Backfire and stutter

guru · 07-06-2008, 11:13 AM

I have a '75 bumped to 2L with webers. Lately she's been backfiring and stuttering. The backfiring occurs only when I floor the gas pedal . i cleaned up the gastank, replaced the rubber fuel lines and put a new filter. After that she doesn't backfire as often as she used to. But there is still the ocassional stutter. It happens while idling and also happens while driving but not so frequent. The tach reacts when the stutter occurs while idling ( can't notice it while diving ). I read in one of the posts that if the tach reacts it could be ignition related. I had replaced the rotor and my distributor has a compuwire ignition, so no points.

I'm thinkin that the backfire and the stutter might be two different problems just happened to occur together? My carbs are running really rich so that could be a reason for the backfire, though it stumps me as to why it did happen before. I'm thinkin of getting a weber manual and try to tune em ( i'm getting only around 15 mpg)

Any suggestions as to where the problem might be for the stutter?

Tobra · Tobra's Garage

if you are getting 15 mpg, and the ignition is okay, your carbs are jacked up

Tobra · Tobra's Garage

if you are getting 15 mpg, and the ignition is okay, your carbs are jacked up, may be timing too

david3299 · 08-11-2008, 07:04 PM

I have a 1987 Porsche 930. Today, on my home the car started to backfire and stutter. I found a loose distributor coil and the + terminal was shorted to the chassis of the car.

PROBLEM SOLVES.

guru · 08-18-2008, 12:00 PM

ok so i've been doing some stuff and now it looks like i have a consistent set of problems.

I replaced the fuel pump up front and installed a facet barrel chaped fuel pump that pumps 4psi max. I also replaced the spark plugs and put in iridium platinum ones.

The backfire is still there and I think I'll have to tune the carbs to get trid of that. But now when I floor the gas pedal, I suddenly lose power. It feels as if the spark plugs are not firing properly and I thought they might be getting fouled out since my carbs are runnin really rich. Hence the switch t iridium plugs, and one level hotter than stock. But it still doesn't solve the problem.Now for more observations on this problem
- It doesn't happen if i drive without flooring the pedal, lets say if I depress the pedal only till half way
- If floor the pedal, the car starts stuttering.
- After that happens, if I slow down and use the gas pedal only slighly, it doesnt stutter. But if i depress it more than say 2 inches, it starts stuttering, ease it and it disappears, depress it a lil more and it happens again.
-I thought it was dependent on rpm but its not, the stutter seems to be dependent on the throttle position.
-Once the car cools down then its all fine and then all i Have to do to bring it back is go hard.

I realize that 'stuttering' might be a a different problem, but it does feel and sound like a stutter. And a consistent stutter too. I dont remember when it started. My carbs were always running too rich. I still need time b4 I can get to tuning it. And the only major work done just prior to it happening was replacing the fuel lines and fuel filter. The fuel pump was replaced just recently.

Amy pointers would be greatly appreciated!!

Joe Ricard · Joe Ricard's Garage

You have to over lapping problems with your carbs

Studdering and popping is lean. happens on the acceleration circuit. you need more gas there. Depending on early vs late carbs the fix is different.

no power and bogs with fouled plugs when wide open throttle. is too rich and you need to go down in jet size. for a 2.0L I would guess at 135 main gas 200 air correction.
50 idle jet.
Next is to get rid of those crappy fancy schmancy plugs.
NGK Copper B6ES or B7ES. or a Bosch Copper of similar heat range. PERIOD!

Joe Ricard · Joe Ricard's Garage

You have to over lapping problems with your carbs

Studdering and popping is lean. happens on the acceleration circuit. you need more gas there. Depending on early vs late carbs the fix is different.

no power and bogs with fouled plugs when wide open throttle. is too rich and you need to go down in jet size. for a 2.0L I would guess at 135 main gas 200 air correction.
50 idle jet.
Next is to get rid of those crappy fancy schmancy plugs.
NGK Copper B6ES or B7ES. or a Bosch Copper of similar heat range. PERIOD!

guru · 08-19-2008, 11:25 AM

Thanks Joe. My carbs are dual Weber IDF 40's. And it was a stock 1.8L bumped to 2.0L.

I dont think fouled plugs could be the problem. Because right after I replaced the spark plugs I took it for a test ride. Shifted to first, floored the pedal and there the stutter/chugging happened. So can new plugs fouls in less than 5 secs?

So I was going through the forums looking for problems similar to mine and I think stutter might not be the right word to describe. I know there's definitely no popping. It more like chugging but like fast chugging. And the poser loss is definitely due to that chugging, you would think that the cylinders are not firing in one continuous cycle and so it chugs instead of a smooth note.

But I think its time I got to retuning the carbs and getting rid of thr richness before I try anything else

Joe Ricard · Joe Ricard's Garage

Ah,
This was something I found with one of my carbs. One of the Main jet holders was not screwing in all the way. Had a burr on the threads. Never caught it till one day I was noticing that one jet stack was a bit higher thanthe other. Changed out the holder and fixed it.
What was happening was that particular cylinder was not picking up the same amount of gas as the other 3.

You could just be not making enough spark. Should be big blue spark when plug is grounded to the engine case.
Also check the ground at the transmission as it is the path of electricity for good spark.

I added a ground wire from each head to the chassis. But I'm throwing 50,000 volts at my plugs.

MarkusB · 08-30-2008, 11:32 PM

I'll bet if you go back to stock plugs you'll notice a difference. Our little cars don't do well with the platinums for some reason.

guru · 09-08-2008, 05:55 PM

I forgot to mention that the exhaust pipes on one side broke off the muffler. Would this have anything to do with the issue I'm seeing?

Kevin Powers · 09-13-2008, 07:03 AM

verify the facet pump truely produces 4lbs of fp with a gauge. 4 is a tad high. my guess is it may be higher. try bosch w7cc or w8cc plugs with the correct gap. fix the exhaust, verify the timing, and valve adjustment. how old are your plug wires? the stutter may be due to an out of adjustment linkage. check the carb flow (equal on all 4 venturis) with your unisyn. go thru the pre-install setup proceedure (fully detailed in your cbperformance weber book).