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Back firing through #1,2 Carb

FM Former Member
I just installed a new 1 5/8" exhaust on my 1835 type 1. I started the car and all was well. Moments later, I restarted the car and noticed a loud noise coming from the left side of the engine, (mine is a mid engine), My first assumption was an exhaust leak at the #2 port. It had that basic sound. After replacing the gasket and retorqueing the nuts. I started the car and still had the sound. As I payed closer attention, I noticed the noise was coming through the left carb. As I gave it throttle, it would backfire loudly through the left carb only. Right side is smooth and quiet. I let the car cool for a few hours and looked into the valvetrain expecting to find a stuck valve. All valves cycled well. I check for clearance and found both exhaust and intake on #1 to be a few thousandths tight. #2 was pretty good on both. I did not find a broken valve spring. I even backed the adjuster off of both intake valves one at a time to see if it stopped the backfire through the carb. The only thing I found was my plugs were gapped to large. The vehicel ran great last year before I put her to bed for winter. It was cold blooded and would backfire a bit if accelerated cold.

It would also appear that this engine is running hydraulic lifters as I can back the rocker off the valve about 3/16"-1/4" with a bit of pressure.

I can't imagine any carb issue just showing up. Ignition timing issues would'nt seem to only show up on one side. Plug wires are new as is cap and rotor. Any ideas before I have to go tearing into it?

Thanks for the ideas.
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FM Former Member
I just installed a new 1 5/8" exhaust on my 1835 type 1. I started the car and all was well. Moments later, I restarted the car and noticed a loud noise coming from the left side of the engine, (mine is a mid engine), My first assumption was an exhaust leak at the #2 port. It had that basic sound. After replacing the gasket and retorqueing the nuts. I started the car and still had the sound. As I payed closer attention, I noticed the noise was coming through the left carb. As I gave it throttle, it would backfire loudly through the left carb only. Right side is smooth and quiet. I let the car cool for a few hours and looked into the valvetrain expecting to find a stuck valve. All valves cycled well. I check for clearance and found both exhaust and intake on #1 to be a few thousandths tight. #2 was pretty good on both. I did not find a broken valve spring. I even backed the adjuster off of both intake valves one at a time to see if it stopped the backfire through the carb. The only thing I found was my plugs were gapped to large. The vehicel ran great last year before I put her to bed for winter. It was cold blooded and would backfire a bit if accelerated cold.

It would also appear that this engine is running hydraulic lifters as I can back the rocker off the valve about 3/16"-1/4" with a bit of pressure.

I can't imagine any carb issue just showing up. Ignition timing issues would'nt seem to only show up on one side. Plug wires are new as is cap and rotor. Any ideas before I have to go tearing into it?

Thanks for the ideas.
FM Former Member
Think """Dirty Carb's"""" Dirt stuck in the idle jet can do several things, including popping and back firing through the carb. It can give you the impression that the problem is electrical and or mechanical (ala valves)

I suggest you start by removing the idle jets. Take a twist tie and strip off some of the paper and use the exposed wire to clean out the jets. Use compressed air (From the small end of the jet only) if you have it available or brake clean..

You said you couldn't imagine any carb issue showing up but I'd bet money that the problem is induction related.

If the car has been sitting for an extended period of time, I always recommend that you turn off your fuel pump (if you have electric) and run the carbs dry before storage. In some cases, people have simply put a turn off valve in the fuel line. Fuel sitting inside the carbs, for the winter, will begin a metamorphous where in it starts life as perfectly good gasoline and by the end of the winter, it becomes crap. Next year, take my advice and run the carbs dry before storage but this year, give my suggestion a try and clean out the idle jets.
FM Former Member
Hi Larry:

I'll give that a try. The other reason why I didn't seem to think carb was that, even at idle, the 1,2 bank was definitely louder than the other. That noise comes through the carb. That is what led me to look at valves right away.

Does that extra bit of info alter your thoughts at all? Either way, I will take your advice on running them dry next year.

Thanks

Dale
FM Former Member
Dale, there is nothing logical about Carbs. Obviously not being there and seeing and hearing is a disadvantage but,, I'd give my suggestion a try. If nothing else, it's part of the "process of elimination game"

Back towards the end of the 70's I worked a multi-jurisdictional task force with a Dale Frye, he worked West Covina PD or Baldwin Park PD, can't remember. I think he resigned and moved to Colorado???
FM Former Member
Hi Larry:

I gave it a shot. The jet was clean no residue on my tip cleaner at all. I sprayed it off with brake cleaner and hit it with the airhose for good measure. Still popping. I ran it a few minutes with the aircleaner off. The carb is definitely pressurized...even when not backfiring. I elevated the idle just a touch and you could see white smoke puffing out. Hold your hand over it and you can feel the pulses. Of course, as soon as I cracked the throttle I'd get a big pop.

It sure seems like the intake valve isn't seating on either 1 or 2. But I find no evidence of that when I measure spring heights. Admittedly, getting accurate heights is difficult with calipers measued against the casting.

I'm thinking I need to do a pressure check on the cylinders.

Again, I'm open for opinions and ideas. Thanks!

Dale
FM Former Member
Hey Larry:

I did a pressure check and came up with:
#1 120psi plug: New sooty
#2 150psi plug: New sooty
#3 150psi plug: New sooty
#4 160psi plug: New clean and brown

Plugs sure look ugly having only about 20 minutes on them. They sure seem to indicate an induction issue.

Pressure is down on #1 but not so much that I'd expect a valve issue


I'm only running dual 32 ICT's. I'm a bit surprised at the plugs.

Ideas?

Thanks
Dale
FM Former Member
Initially you stated that the engine may be equipped with hydraulic lifters??? Have you confirmed that? Although the engine has hydraulic lifter, an initial valve setting is required, are they set correctly? Maybe a lifter is pumping up before others and changing the valve clearance??

That 120 reading is kind of low compared the the 150 cylinders, in fact, that one clinder is 20% below the rest.
Ok, I know this may sound strange but,,, start the engine and let it gather some heat.

Take a glass of water and slowly pour it down the carb on the 1-2 side as you keep the engines' idle up.
It's going to want to stall but keep giving it gas to keep the engine lit.

Sometimes, carbon can build up on the valves or the seat and cause the valve to stay open a 1/1000 of an in inch, engough that it causes problem and not enough to show a significant difference in spring height.

The water turns to steam and cleans the "innards" of your engine along with the valves and seats, piston tops, rings and cylinders.

8 oz of water should do the trick. Don't be surprised if a whole bunch of white smoce appears from your tail pipe.


FM Former Member
Wow - that will work? Never done that. Have done the ATF in the fuel tank thing, but simple water? COOL Larry! (no pun intended).

Will the water have any effect on the rings? I once torched a set of rings due to an intake manifold/water leak on a fiat motor, but I suspected it was actually the anti-freeze that caused the problems, not the water. Is that correct? angela
FM Former Member
Angela, I was just as incredulous as you. However, my son (24) has an old turbo Mitsi Eclipse and was saying it was making a tat-tap-tappity noise when it started, but went away after warm up.

He said he was going to pour something called Seafoam into the intake and then change the oil on just this past Saturday morning, before he drives 200 miles home for Easter. I said: "You are going to do what?????"

He said: "It's OK, I read about it on DSM Turners (the Mitsi eq. of spyderowners).

Miraculously, he showed up Saturday morning on schedule. He said the huge smoky exhaust show thrilled his neighbors, and that he doesn't remember the car running as well as it does now. I happened to notice the exhaust pipe tips appeared to have been scoured clean of soot.

Incientally, the tap-tap-tappity noise is was gone when he started the engine cold.

I guess it works -- at least for 200 miles' worth.

Sooo . . . . Angela, as soon as you get that new engine plunked into the end of Spitter; go get the garden hose and . . . .

Mark
FM Former Member
Hi Larry:

It definitely actes as if they are hydraulic. After the engine sits and cools a bit, you can pull on the rocker arm to gain an additional 3/16 - 1/4 inch clearance between the rocker and valve. However, with engine cold, I went through and checked clearances. There were a couple that were a few thou tight. I am using .008" as my heads have the studs protruding through as described in my ol' Chiltons' Beetle Book.

I have done the water trick before...one of those old school tricks that I had forgotten about. The engine is only supposed to have 8K miles on it. So, I wouldn't normally expect that much carbon yet. However, as sooty as the plugs read, it is certainly a possibility.

The soot also has me surprised as this motor is only running 32ICT's on an 1835cc motor. I do not know the jetting. I would have, however expected a leaner condition with the new better flowing "Vintage Spyders" exhaust.

Thanks Again

Dale
FM Former Member
Hey Larry

She's running! The water got me a long way. It evened the engine out enough to tell that I had a bit too much spark advance as well. A few degrees of retard and we're back in business. Well, that was until the cable stop on my throttle linkage broke. lol It's always something isn't it!

Thanks a bunch for your help!

Dale
FM Former Member
I'm glad it worked, good luck

In regards to your spark, With the timing light attached, rev up the engine and hold it at about 2800 to 3000 RPM's.

Maximum advance should be no more than 28 degrees BTDC. If you're running racing gasoline (leaded 100+ octane) you can handle up to about 32 degrees.

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