backfiring through carbs Options

[last337]

I am having a hard time with my engine backfirin when I'm cruising in say 3rd and let off the gas. I also tend to feel like im run very rich when I take off from a start. I'm confused

[McMark]

'Bout $200 gets you a wideband O2 sensor and takes all the guesswork out of tuning. (IMG:style_emoticons/default/wink.gif)

[last337]

So here is an update. Pulled all plugs and they look perfect. Started with initial setting on carbs according to Tomlinson and ran perfect then took off. Got a few blocks and started dying completely. Got back to shops barely and let it sit then started and idled perfect again. Seems like fuel for sure but pump is working well and filter Clea . Any ideas?

[pcar916]

Since you haven't told us your pump diagnostics. I've had several cases over the years when when what you describe still turns out to be the pump. Flow measurements (engine not running) were fine and so was the pressure, but while running, the pump couldn't keep up so the dynamic flow at pressure was bad...

Not saying this is the problem but just a thought.

Good luck

[last337]

Well I was just messing with it more and had it tunes perfect or so I thought then went to make another pass and one carb barely responded then I throttled up and then let it fall off again and it responded again. I think you may be right. Maybe I need to bite bullet and get a new pump. Any suggestions?

[pcar916]

If you can find one locally to test it, that's ideal. Buying a new/rebuilt one for a test will be expensive if it turns out to be something else. Find a buddy nearby or someone here who can lend you one for a test. I'd go with a Facet rotary

http://www.racerpartswholesale.com/product...CFU6f4Aodr2gAXg

but the diaphram pump works well with carbs also.

http://www.racerpartswholesale.com/categor...et_Fuel_Pumps_2

With the pressure at about 3-5 lbs either is a solid pump.

Good luck

[last337]

I just put on a brand new facet rotary that is giving me the problems. I think it may be something else although I have seen others say their pumps dies quick.

[pcar916]

Those pumps are outstanding as long as the pressure is regulated well. I've never had a defective one. Backfiring through the carbs is usually a lean mixture problem. I suppose I should say this can be either fuel starvation or a vacuum leak.

[last337]

Well now it is backfiring through exhaust at low load even taking off. I am wondering if I have a vacuum leak or something.

[McMark]

Did you set the float heights? The 'high' setting is a little tricky because there is a spring in the needle valve. You don't want to compress the spring when measuring the 'high' side. If you do it will be too low in actuality.

Do you have a regulator on the pump?

Did you sync the carbs? Did you sync them off-idle?

[rhodyguy]

can you tell which side is snapping back thru the carb? take the filter tops off and increase the engine speed by hand. you should be able to note the offending carb. DO NOT look down the carbs when you perform this!

a facet and a rotary are dif animals. if you're running a facet and not using a fp regulator and a gauge to verify the pressure you're spinning your wheels.

so...you've achieved a stable idle, both carbs flowing the same @ idle, and it all goes to crap when you start to open the the throttle plates. yes? quick test. at idle, linkage connected, back the PASS side idle speed screw off the stop. use the drivers side idle speed screw to increase the engine speed to 1700rpm or so. the progression circuits will now be coming into play (the circuits are shown and explained in the weber manual p.57). now remeasure the flow with the linkage connected. if the readings now differ, the lowest reading carb is playing 'catch up' and will be leaner than the other one. reading the same? report back and we'll see what's next to check. linkage bind can also deliver the back firing on decel.

read and study the trouble shooting guide. there are 8 possible solutions to "misfiring during acceleration".

have you cleaned/replaced the filter sock/screen in the fuel tank prior to installing your carbs?

can you post some pictures of your linkage? we'll need to see the angle of the drop links too.

[michael7810]

Mark - can you elaborate on sync'ing the carbs off idle? I only sync mine at idle. Thanks

[Sneezy]

Clean your idle jets.

[McMark]

Mark - can you elaborate on sync'ing the carbs off idle? I only sync mine at idle. Thanks

Syncing the carbs at idle just gets everything balanced at zero throttle. Because of slight play in even the best linkages, you have to open the throttle slightly and hold it steady while looking at airflows with your syncrometer. It's not that weird to have perfectly synced idle and be way off above idle. Slight tweaks in the linkage can get you to a perfect idle/off-idle sync. Sometimes (with a cross bar linkage) we're talking about 1/4 turn or less.

If you can't get a perfect idle/off-idle sync, you may have a problem with your linkage where the rate of opening is different from one carb to the other. In this case you need to spend time looking at the linkage arms and other pieces besides the threaded rods. Cross bar linkages are the most likely for this sort of trouble.

[last337]

I thought a facet was already low pressure. How can you regulate fuel pressure on such a low pressure pump? I would think you would need at least a few psi difference to get a regulator to even open up.

[pcar916]

You have to include a regulator inline with the pump between it and the carbs. Otherwise the pressure will be too high, especially with the rotary pump. Add a regulator dude! Even if that isn't your only problem you'll have to eliminate that first.

Mine would regulate between 1 and 5 psi. Here's a link to another regulator but it's by no means the only one. The second link is to the Syntec I used with the 40IDA3C's on a 2.7L engine that came in my car when I bought it.

http://www.racerpartswholesale.com/product...el_System_Other

http://www.fuelpumpsonline.co.uk/sytec-flo...1-5psi-30-p.asp

Your carbs will be happy between 3 and 5 psi but might object to higher pressures. Carburetor's float valves generally like that range.

[Rand]

I thought a facet was already low pressure. How can you regulate fuel pressure on such a low pressure pump? I would think you would need at least a few psi difference to get a regulator to even open up.

Pressure is pressure. The regulator doesn't need more psi than you want to pass. If the pump makes 5lbs and your carbs want 4, the regulator works. Is that not your experience?

[last337]

Well I'd say I've uncovered my problem. I pulled one whole carb apart and as I was doing so I noticed the jets were all wrong. The main jet is only a 115 and the idle is only a 52. I think the first thing I should do is start with changing those. Tomlinson says either a 145 or 155 for main. What do you guys think for a 2.0? Also where is a good place to buy jets?

[last337]

I thought a facet was already low pressure. How can you regulate fuel pressure on such a low pressure pump? I would think you would need at least a few psi difference to get a regulator to even open up.

Pressure is pressure. The regulator doesn't need more psi than you want to pass. If the pump makes 5lbs and your carbs want 4, the regulator works. Is that not your experience?

Well I work in natural gas and my experience is mainly with those regulators. With a ng regulator u need enough of a differential between upstream and downstream for a regulator to work properly

[BarberDave]

(IMG:style_emoticons/default/smilie_pokal.gif)



Pull the idle jets and blow them out. Mine run like shit untill off the idle circut, about 3,000

RPM in hight gear. After that run perfect. That became part of my reg. maintance

, sometimes 1 or twice a yr. Dirt? old gas ? Takes 5 mins. Dave (IMG:style_emoticons/default/slap.gif)