Callin the Cap'n or other engine Yodas, Manifold vacuum gauge readings Options

[ThePaintedMan]

I took the advice of a few folks on here and finally got around to buying a quality vacuum gauge. As a primer, I've had all kinds of issues, from carb synching, timing, etc. But I think I've finally got it running fairly well. The car is a '73 with a 1.7, dual Weber 40 IDFs.

Anyhow, I did a compression test the *right* way this time with the throttle open and after the engine was warm and got 110 across the board. Not great, but not awful. Next I moved onto checking the vacuum on each manifold port at the base of the carbs. I get wildly, but consistently fluctuating numbers on each cylinder. By that I mean, at idle, each cylinder displays the same thing - the needle bounces from 5-17 mm Hg instead of a steady number. I've been reading up on this at:

http://www.thesamba.com/vw/forum/viewtopic.php?t=476817

and the great animations at:

http://www.therangerstation.com/Magazine/S...VacuumLeaks.htm

Is it then possible that I have FOUR burned exhaust valves? I had been suspecting I had at least one for awhile, due to seemingly high CHTs on #3, "sucking" coming from the tailpipe and the fact that #3 doesn't suck nearly as much air from the velocity stack as the others. But now I'm suspecting a larger issue. Is it possible that both heads are loose? I see some oil leaking from the cylinder-case junction, but don't see anything from the head-cylinder junction. Would it be worth taking the in off and trying to retorque all the head studs, if nothing else, for piece of mind?

-George

[brant]

is this a high duration camshaft?

there can be valve overlap in a high duration camshaft
that is why a fuel injected car can't handle anything but a very mild camshaft....


how about balancing the carbs?
did you use a sync gauge to go through the balance procedure
brant

[wndsrfr]

You're going to get a bouncing vacuum signal on any individual cylinder just due to the stop/go nature of the intake pulse. If you want to check for a burnt valve, do a leakdown test--that'll tell you what you want to know. Bring your piston to TDC for each cylinder, put compressed air on the spark plug port and listen for leakage in the intake and exhaust and also in the oil fill opening.....

[Dave_Darling]

The instructions you're following are for a system that has a single plenum that feeds all of the cylinders. Like the stock fuel injected system does. With the Webers, you're only seeing the vacuum for one single cylinder, and that will vary a lot. If they all behave the same way across the board, chances are that the vacuum reading is normal, or at least close to it.

--DD

[Mike Bellis]

You need an "inches of water" vacuum gauge. A regular gauge will not work to sync carbs as well.

[rhodyguy]

george, the bouncy needle at idle is to be expected. pm sent.

[ThePaintedMan]

I feel like all of my posts end up being "doomsday scenarios," lol. Or that I keep putting too much information in a post at once. Once I went back and read it I'm sure it was confusing for you guys. Very sorry, but always grateful for your help!

As far as your responses, yes, I used a "snail" gauge to sync the carbs. I think the throttle shaft on the drivers side carb is a little bent since when the other passenger carb is completely at idle-rest, I can still pull on the driver arm and get it to idle down some more. For the meantime, I have been synching them at higher RPMs since this is where most of the driving is done anyway.

Regarding the bouncing signal, that makes a lot of sense. I should have known the instructions I was going by were not based off of a single-throttle-body application. My mistake.

This still doesn't quite explain why cylinder #3 doesn't draw the same numbers as the rest of them, though. I'm curious what that might be about. I read another post awhile back with someone having a similar issue, but never heard any real ideas on it, other than maybe retorquing the heads.

It is very possible that this engine has a carb cam in it, but I have no way to verify it. I got no information on whether it was rebuilt by the previous owner(s), but I was under the impression it was original. Its obviously lived a hard life, but its taught me a lot and got my ass to and from work all summer. I have been speaking with Jon (no1uno) over in Orlando about a rebuilt 1.8 he has to swap out eventually. I'm just not quite ready to give up on this one yet (IMG:style_emoticons/default/smile.gif)

Thanks for your help and continued tutelage! (IMG:style_emoticons/default/beerchug.gif)

[Madswede]

is this a high duration camshaft?
there can be valve overlap in a high duration camshaft
that is why a fuel injected car can't handle anything but a very mild camshaft....

(IMG:style_emoticons/default/huh.gif) Oh, you mean stock FI, of course.

[Elliot Cannon]

I feel like all of my posts end up being "doomsday scenarios," lol. Or that I keep putting too much information in a post at once. Once I went back and read it I'm sure it was confusing for you guys. Very sorry, but always grateful for your help!

As far as your responses, yes, I used a "snail" gauge to sync the carbs. I think the throttle shaft on the drivers side carb is a little bent since when the other passenger carb is completely at idle-rest, I can still pull on the driver arm and get it to idle down some more. For the meantime, I have been synching them at higher RPMs since this is where most of the driving is done anyway.

Regarding the bouncing signal, that makes a lot of sense. I should have known the instructions I was going by were not based off of a single-throttle-body application. My mistake.

This still doesn't quite explain why cylinder #3 doesn't draw the same numbers as the rest of them, though. I'm curious what that might be about. I read another post awhile back with someone having a similar issue, but never heard any real ideas on it, other than maybe retorquing the heads.

It is very possible that this engine has a carb cam in it, but I have no way to verify it. I got no information on whether it was rebuilt by the previous owner(s), but I was under the impression it was original. Its obviously lived a hard life, but its taught me a lot and got my ass to and from work all summer. I have been speaking with Jon (no1uno) over in Orlando about a rebuilt 1.8 he has to swap out eventually. I'm just not quite ready to give up on this one yet (IMG:style_emoticons/default/smile.gif)

Thanks for your help and continued tutelage! (IMG:style_emoticons/default/beerchug.gif)

FWIW It's been my experience that you absolutely must start carb adjustment with a carb linkage that is solid, with no play whatsoever and the geometry must be set up so the carbs open fully and close fully and they should both open and close at the same rate. Then comes jetting and syncing. I have 44idf's on my 2.2 type IV. I used that setup sequence and the car runs great.

[Elliot Cannon]

This helped me a whole lot. http://carburetion.com/weber/adjust.htm