Backfire Through Carb

kevroo · Post by **kevroo** » Fri Nov 26, 2004 7:37 am

I'm restoring a 73 914 with my son for his first car and have an engine problem I can't figure out. When we purchase this car it was a 1.7L w/ 96mm bore, non-stock cam, 40 IDF Webers, one loose head and it ran poorly. We installed new bearing, 96mm Ps & Cs, had the lifters refaced and 1.8L heads refurbished by Rimco. It ran OK for about ten miles. It didn't have great power; however it rev'd cleanly. Then it started backfiring through the number 3 carb venturi. I have checked the valve clearences several times, replaced everything in the ignition system at least once, rebuilt and replaced one new carb. I have drained the oil through a paint filter and found no metal in the oil, strainer, or filter. The compression is 140 on all cylinders. I have tried to measure the up and down movement of ther rocker arms on #3 cylinder intake and exhaust valves while the engine is being cranked. They may be about 1/16" short of the up and down movement of the # 4 cylinder valves. If a flat cam lobe is causing this backfiring on # 3, are not the same cam lobes also opening and closing #1 cylinder valves? I appreciate your ideas on this problem, I'm stumped.

MASSIVE TYPE IV · Post by **MASSIVE TYPE IV** » Fri Nov 26, 2004 9:25 am

Re check the valves, especially the intake on that cylinder..

Did you do anything to the cam???

Post by **Piledriver** » Fri Nov 26, 2004 2:34 pm

Pull the lifters one at a time and remove all doubt.

If they don't slide right out do NOT force them and hose the case.

Common issue for the last 2 years or so.

Use the forums "search' feature, it's been beaten to death, resurrected, beaten, bla bla bla over many cycles.

MASSIVE TYPE IV · Post by **MASSIVE TYPE IV** » Fri Nov 26, 2004 2:37 pm

chances are the issue is a dead lifter that has not totally wasted the lobe and the opposing lifter- YET..

It could be an exhaust lobe/lifter wiping away not letting the exhaust out all the way and causing the charge to want its way back out on the intake opening event..

Post by **Piledriver** » Fri Nov 26, 2004 6:35 pm

I would suggest again just pulling the lifters: it takes like 20 min, and it sounds like you will need to have a look sooner or later anyway. It hurts NOTHING to look, and will give you peace of mind.

Also search on pushrod tube sealing in the Type4um... Some excellent info there to keep a dry motor.

Rereading your post, one other thing occcured to me--- Does it pop at idle, or under load? (this is assuming the lifters and cam check out...)

If at idle ONLY, you have just picked up some crud in the idle fuel jet, happens all the time, the air hole to those jets is in the perfect place to pick up any crud on top of the carbs.

CB Performance sells a kit that looks like it will greatly reduce that happening. If I didn't have a 914, I would have bought it already.
RTVing the air cleaner elements into the tops, and heavy grease on the bottom reduces this greatly.

In a bug, those jets require an act of god to get out.

kevroo · Post by **kevroo** » Sat Nov 27, 2004 9:10 am

Thank you for the replies. This engine backfires through # 3 carb venturi at an idle and continues backfiring while the engine is reved up. Putting a load on the engine by driving it causes the engine to run really poorly. I initially thought that this indicated an ignition problem due to the noticable deteriation in firing under load.

I also replaced the #3 & 4 carb with a new carb.

I have read quite a bit about Type 4 lifter and cam problems. I had assumed that if a lifter was losing its face, the valve lash would be increasing. I have checked the valves several times and I have only reduced the clearence in # 3 Intake and exhaust by 1 or 2 thousanths total.

Is the only way to pull a lifter without pulling the engine to cut a pushrod tube in half?

Post by **Piledriver** » Sat Nov 27, 2004 1:03 pm

Cut a pushrod tube in half???

This is a T4, not a T1, they slide out thru the valve cover.

On a 914, you might have to support the motor on a jack and let the front mount bar down a few inches. The whole shot is held up with 4 nuts...

(The above just to clear the suspension consoles)

It's harder to explain than do.

If your valve clearance isn't changing, it almost sounds like a bent valve. Or swapped plug leads. Or a cracked cap...
(Ignition problems are "fun" to diagnose if you aren't there...)

914fan · Post by **914fan** » Mon Nov 29, 2004 11:18 am

Check and replace the intake to head gasket. I have blown out 10 so far. The mating surface is thin at the top portion of the intake holes were the factory cooling tin rubbs it between the two ports. The generic gaskets do not allways provide a good seal. I ended up cutting my own out of a 1.5m rubber sheet. Pulling the carb with manifold off and replacing everything can be done in less then 30 min. If you do not change the settings on the linkage you should not need to do much if anything to re-sync the carbs.

Good luck.
/ \
O O
the wear spots are at 1 o'cloc and 11 o'clock on the two ports as shown above.

Post by **Piledriver** » Tue Nov 30, 2004 2:57 pm

If that's the case, you need to sand/lap your manifold bases flat...

Even the crappy single progressive setups seals once flat.

I have lost one of those gaskets in ~20 years on dozens of motors...
(had a progressive//non flat flange issue)

They arent even blown on core teardowns.

914fan · Post by **914fan** » Wed Dec 01, 2004 12:13 pm

Its not the manifold. It is the head. sealing surface it thin between the two ports were the shroud rubbs. The universal gaskets end up having only 1/8 inch or less of surface to seal due to the ports on the gasket being larger than the head. After installing the cut to fit gaskets I have had no problems.

Just something to check. If the gasket is broke or there is not a seal there will be popping through the carb.

Post by **Piledriver** » Wed Dec 01, 2004 3:13 pm

Ah, a case of bad engine tin...

914fan · Post by **914fan** » Thu Dec 02, 2004 10:51 am

! have 7 heads that all have this problem to some degree. I will eventually send a pair to ham and have him work his magic as well as repairing this issue.

kevroo · Post by **kevroo** » Wed Dec 15, 2004 7:46 pm

Well, thanks for the suggestions. So much for an inexpensive rebuild. I get to try it all over again. After 60 miles this engine isn't getting lift on #1 or #3 exhaust valve. The lifter bores seem fine.

Kevroo

MASSIVE TYPE IV · Post by **MASSIVE TYPE IV** » Wed Dec 15, 2004 7:58 pm

Cam is gone....

Welcome to the club and the house of pain..

And people think that my cam and lifter service is expensive!

DO NOT CHEESE OUT ON THE DAMN CAM AND LIFTERS!

Post by **Piledriver** » Wed Dec 15, 2004 11:45 pm

Just get the ceramics... It only hurts once, in the wallet...
(they're reusable as opposed to anything else, and Griznant says he hasn't actualy had to adjust a valve yet)

Any news/updates on the second source, Jake?

I'll be looking for something really light, and if I dont have to bush fo T1s, it'll save me $$200+ shipping...

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