Jetting Webers on 2.7 911 engine., Tuning. Options

[914Toy]

After further research and inspection of the carbs, it seems logical to me, as suggested by some on PP, that the idle air jets are too small, as there is almost no change in A/F's when reducing the idle jets from 60's to 55's, which is actually a significant reduction in jet cross sectional area, about 14%. Thus I am considering drilling these air jets out a little as some others have done. The air jet holes are close to 1.3mm in diameter. What increase in diameter does anyone suggest?

I'm against reaming jets. No matter how careful one is it never seems to work out well in the end.

Your idle jet meters A/F during the progression phase. The mixture screws only adjust the A/F mixture when idling. No other time.

Bigger air corrector brings mains in sooner smaller delays the mains. Best to pull the whole stack and drive with only the idle jets. You'll know exactly when it fall on it's face. Be conservative with the accel pump adjustment during this time.

After further thought and discussion with Paul, I will not drill these air jets. I have now checked the accelerator pumps which are working well, and will acquire a set of "hatchet" cams for the acceleration pumps in an attempt to eliminate the small occasional acceleration hesitation. If that does not eliminate this issue, I will replace the 55' with 60's idle jets.

Thanks for all suggestions made.

[72hardtop]

After further research and inspection of the carbs, it seems logical to me, as suggested by some on PP, that the idle air jets are too small, as there is almost no change in A/F's when reducing the idle jets from 60's to 55's, which is actually a significant reduction in jet cross sectional area, about 14%. Thus I am considering drilling these air jets out a little as some others have done. The air jet holes are close to 1.3mm in diameter. What increase in diameter does anyone suggest?

I'm against reaming jets. No matter how careful one is it never seems to work out well in the end.

Your idle jet meters A/F during the progression phase. The mixture screws only adjust the A/F mixture when idling. No other time.

Bigger air corrector brings mains in sooner smaller delays the mains. Best to pull the whole stack and drive with only the idle jets. You'll know exactly when it fall on it's face. Be conservative with the accel pump adjustment during this time.

After further thought and discussion with Paul, I will not drill these air jets. I have now checked the accelerator pumps which are working well, and will acquire a set of "hatchet" cams for the acceleration pumps in an attempt to eliminate the small occasional acceleration hesitation. If that does not eliminate this issue, I will replace the 55' with 60's idle jets.

Thanks for all suggestions made.

Down on the idle jets. After LBI you should be ~14:1 AFR

One can be really rich with a 40 idle jet or really lean with a 60. It's all in the AFR screw adjustment. Remember the A/F mixture screw will ONLY adjust the mix for idle speed setting. Nothing else.

Your progression phase is dictated via the idle jet size which is already pre-mixed via the air coming from the air bleed port/s.

[914Toy]

After further research and inspection of the carbs, it seems logical to me, as suggested by some on PP, that the idle air jets are too small, as there is almost no change in A/F's when reducing the idle jets from 60's to 55's, which is actually a significant reduction in jet cross sectional area, about 14%. Thus I am considering drilling these air jets out a little as some others have done. The air jet holes are close to 1.3mm in diameter. What increase in diameter does anyone suggest?

I'm against reaming jets. No matter how careful one is it never seems to work out well in the end.

Your idle jet meters A/F during the progression phase. The mixture screws only adjust the A/F mixture when idling. No other time.

Bigger air corrector brings mains in sooner smaller delays the mains. Best to pull the whole stack and drive with only the idle jets. You'll know exactly when it fall on it's face. Be conservative with the accel pump adjustment during this time.

After further thought and discussion with Paul, I will not drill these air jets. I have now checked the accelerator pumps which are working well, and will acquire a set of "hatchet" cams for the acceleration pumps in an attempt to eliminate the small occasional acceleration hesitation. If that does not eliminate this issue, I will replace the 55' with 60's idle jets.

Thanks for all suggestions made.

Down on the idle jets. After LBI you should be ~14:1 AFR

One can be really rich with a 40 idle jet or really lean with a 60. It's all in the AFR screw adjustment. Remember the A/F mixture screw will ONLY adjust the mix for idle speed setting. Nothing else.

Your progression phase is dictated via the idle jet size which is already pre-mixed via the air coming from the air bleed port/s.

Issue is with 55's and mixture screws turned out 1/2 turn, I have smooth best idle speed at 800 rpm, but A/f's of 10.5. Checked throttle plates at this idle are above the idle port and below the transition ports - where they should be. I am suspicious of the absolute accuracy of the A/F meter, which is consistent but may not be factory calibrated correctly.

[72hardtop]

After further research and inspection of the carbs, it seems logical to me, as suggested by some on PP, that the idle air jets are too small, as there is almost no change in A/F's when reducing the idle jets from 60's to 55's, which is actually a significant reduction in jet cross sectional area, about 14%. Thus I am considering drilling these air jets out a little as some others have done. The air jet holes are close to 1.3mm in diameter. What increase in diameter does anyone suggest?

I'm against reaming jets. No matter how careful one is it never seems to work out well in the end.

Your idle jet meters A/F during the progression phase. The mixture screws only adjust the A/F mixture when idling. No other time.

Bigger air corrector brings mains in sooner smaller delays the mains. Best to pull the whole stack and drive with only the idle jets. You'll know exactly when it fall on it's face. Be conservative with the accel pump adjustment during this time.

After further thought and discussion with Paul, I will not drill these air jets. I have now checked the accelerator pumps which are working well, and will acquire a set of "hatchet" cams for the acceleration pumps in an attempt to eliminate the small occasional acceleration hesitation. If that does not eliminate this issue, I will replace the 55' with 60's idle jets.

Thanks for all suggestions made.

Down on the idle jets. After LBI you should be ~14:1 AFR

One can be really rich with a 40 idle jet or really lean with a 60. It's all in the AFR screw adjustment. Remember the A/F mixture screw will ONLY adjust the mix for idle speed setting. Nothing else.

Your progression phase is dictated via the idle jet size which is already pre-mixed via the air coming from the air bleed port/s.

Issue is with 55's and mixture screws turned out 1/2 turn, I have smooth best idle speed at 800 rpm, but A/f's of 10.5. Checked throttle plates at this idle are above the idle port and below the transition ports - where they should be. I am suspicious of the absolute accuracy of the A/F meter, which is consistent but may not be factory calibrated correctly.

A larger idle will require less turns out from lightly seated position. Go down on the idle jet size to 50 and start with the screws 2 turns from lightly seated position. Then LBI each screw.

[914Toy]

After further research and inspection of the carbs, it seems logical to me, as suggested by some on PP, that the idle air jets are too small, as there is almost no change in A/F's when reducing the idle jets from 60's to 55's, which is actually a significant reduction in jet cross sectional area, about 14%. Thus I am considering drilling these air jets out a little as some others have done. The air jet holes are close to 1.3mm in diameter. What increase in diameter does anyone suggest?

I'm against reaming jets. No matter how careful one is it never seems to work out well in the end.

Your idle jet meters A/F during the progression phase. The mixture screws only adjust the A/F mixture when idling. No other time.

Yes, I have considered and will source some 50's to try this. Will report back.

Bigger air corrector brings mains in sooner smaller delays the mains. Best to pull the whole stack and drive with only the idle jets. You'll know exactly when it fall on it's face. Be conservative with the accel pump adjustment during this time.

After further thought and discussion with Paul, I will not drill these air jets. I have now checked the accelerator pumps which are working well, and will acquire a set of "hatchet" cams for the acceleration pumps in an attempt to eliminate the small occasional acceleration hesitation. If that does not eliminate this issue, I will replace the 55' with 60's idle jets.

Thanks for all suggestions made.

Down on the idle jets. After LBI you should be ~14:1 AFR

One can be really rich with a 40 idle jet or really lean with a 60. It's all in the AFR screw adjustment. Remember the A/F mixture screw will ONLY adjust the mix for idle speed setting. Nothing else.

Your progression phase is dictated via the idle jet size which is already pre-mixed via the air coming from the air bleed port/s.

Issue is with 55's and mixture screws turned out 1/2 turn, I have smooth best idle speed at 800 rpm, but A/f's of 10.5. Checked throttle plates at this idle are above the idle port and below the transition ports - where they should be. I am suspicious of the absolute accuracy of the A/F meter, which is consistent but may not be factory calibrated correctly.

A larger idle will require less turns out from lightly seated position. Go down on the idle jet size to 50 and start with the screws 2 turns from lightly seated position. Then LBI each screw.

[914Toy]

[quote name='914Toy' date='Oct 13 2018, 10:26 AM' post='2656998']
[quote name='72hardtop' post='2656964' date='Oct 13 2018, 07:54 AM']
[quote name='914Toy' post='2656955' date='Oct 13 2018, 07:31 AM']
[quote name='72hardtop' post='2656881' date='Oct 12 2018, 09:32 PM']
[quote name='914Toy' post='2656698' date='Oct 12 2018, 07:49 AM']
[quote name='72hardtop' post='2656524' date='Oct 11 2018, 02:40 PM']
[quote name='914Toy' post='2656463' date='Oct 11 2018, 11:43 AM']
After further research and inspection of the carbs, it seems logical to me, as suggested by some on PP, that the idle air jets are too small, as there is almost no change in A/F's when reducing the idle jets from 60's to 55's, which is actually a significant reduction in jet cross sectional area, about 14%. Thus I am considering drilling these air jets out a little as some others have done. The air jet holes are close to 1.3mm in diameter. What increase in diameter does anyone suggest?
[/quote]

I'm against reaming jets. No matter how careful one is it never seems to work out well in the end.

Your idle jet meters A/F during the progression phase. The mixture screws only adjust the A/F mixture when idling. No other time.

Yes, I have considered and will source some 50's to try this. Will report back.

Bigger air corrector brings mains in sooner smaller delays the mains. Best to pull the whole stack and drive with only the idle jets. You'll know exactly when it fall on it's face. Be conservative with the accel pump adjustment during this time.
[/quote]

After further thought and discussion with Paul, I will not drill these air jets. I have now checked the accelerator pumps which are working well, and will acquire a set of "hatchet" cams for the acceleration pumps in an attempt to eliminate the small occasional acceleration hesitation. If that does not eliminate this issue, I will replace the 55' with 60's idle jets.

Thanks for all suggestions made.
[/quote]

Down on the idle jets. After LBI you should be ~14:1 AFR

One can be really rich with a 40 idle jet or really lean with a 60. It's all in the AFR screw adjustment. Remember the A/F mixture screw will ONLY adjust the mix for idle speed setting. Nothing else.

Your progression phase is dictated via the idle jet size which is already pre-mixed via the air coming from the air bleed port/s.
[/quote]

Issue is with 55's and mixture screws turned out 1/2 turn, I have smooth best idle speed at 800 rpm, but A/f's of 10.5. Checked throttle plates at this idle are above the idle port and below the transition ports - where they should be. I am suspicious of the absolute accuracy of the A/F meter, which is consistent but may not be factory calibrated correctly.
[/quote]

A larger idle will require less turns out from lightly seated position. Go down on the idle jet size to 50 and start with the screws 2 turns from lightly seated position. Then LBI each screw.
[/quote]

Yes - i will try 50's and keep you posted.

[914Toy]

Update.

Installing "hatchet" shaped accelerator pump cams has solved the hesitation in acceleration issue

I have today replaced the 55's with 50's idle jets and this has increased the idle A/F's from 10.3 to 11.5 at 800 rpm's - a good improvement. A test drive and seat-of-the-pants judgement, there is no noticable change in power in the midrange mode with no change in WOT mode. Will stay with this set up and see what miles per gallon I get.

[larryM]

y'all may also find this thread relevant

http://forums.pelicanparts.com/911-engine-...arb-tuning.html

[72hardtop]

Update.

Installing "hatchet" shaped accelerator pump cams has solved the hesitation in acceleration issue

I have today replaced the 55's with 50's idle jets and this has increased the idle A/F's from 10.3 to 11.5 at 800 rpm's - a good improvement. A test drive and seat-of-the-pants judgement, there is no noticable change in power in the midrange mode with no change in WOT mode. Will stay with this set up and see what miles per gallon I get.

Down on the idle to 47.5. LBI should be ~ 14:0

[914Toy]

Update:

I installed the "hatchet" cams on the carb accelerator pump linkages - hesitation is eliminated!! Also installed 50's idle jets - too lean. Back to 55's and engine is running smoothly under all rpm's and varying power ranges.

A/F's still at 10.5 at idle at 800 rpm Have ordered a set of metric jet reamers - two weeks delivery. Will then ream out the small pressed in idle air jets 0.1 mm and report back.

[JmuRiz]

Good to know, never heard of the hatchet cam, I'll look into it if I have issues when mine is installed/tuned.

[914Toy]

Update:
After much reading and testing ideas to discover how to increase my idle A/F's up from 10 - 10.5, including increasing the idle air jet orifices from 1.10mm (stock) to 1.18mm, I found the cause of my issue yesterday.
Previously I had found best idle setting with the idle mixture screws turned completely in (basically closed)? This means that fuel must be entering the throttle bodies from the intermediate orifices in order for the engine to idle. However after several inspections, I could see that the throttle plates were correctly positioned at idle, with their edges covering the lowest intermediate fuel orifice.
Cutting a long story short, with the carbs back on my bench, I observed that the idle mixture screws with "cup"washers covering O rings restricted the screws from being fully screwed in, leading to the previous conclusion that the screws were fully screwed in - closed. After removing the washers and O rings, the screws could be screwed in an additional approximately two turns! I reassembled the carbs and installed them with the mixture screws turned open one full turn. After unsuccessfully attempting to start the engine, I opened the mixture screws one half turn and the engine started immediately. After adjusting further to best idle and allowing the engine to warm up, the idle A/F mixture gauge showed steady 13's - success!!
I have since learned that my Weber 40IDA's were original equipped with mixture screws that did not have these cup washers and O rings. Here are two pics of the mixture screws:

[gereed75]

Hmmmm....very interesting and extreme diligence on your part. I will definitely check mine, as I also have low AFR at idle and do not get consistent LbI indications. Thanks and well done !

[mb911]

Interesting.. I actually need a few of those cup washers. Anyone know where to get them?

[TravisNeff]

You can find a lot of the little parts at partsklassic.com

[914Toy]

Interesting.. I actually need a few of those cup washers. Anyone know where to get them?

I do not believe you NEED them, other than for originality optics purposes. They have no useful function other than perhaps a little help to prevent the screws vibrating loose - unlikely with the springs doing this. The possibility of the intake vacuum pulling air through the threads is minuscule and I would judge as extremely unlikely be enough to have any performance impact. One source suggests these O rings were an attempt to show good faith to environmentalists - per discussion on PP (IMG:style_emoticons/default/smoke.gif)

[72hardtop]

FYI....

There are also 2 types of idle jets used in the IDF's.

Are you sure your using the correct set for your IDF's?

The spring should compress and allow for the idle mix screw to bottom out. Only turn them in until they lightly seat. Don't go full gorilla with them.

[Mark Henry]

I'm running same, just a bit bigger main jet on my 3.0, I also say leave it alone.
Getting better AFR from this point will be a game of a dog chasing it's tail.
Aircooleds need it a hair rich anyways.