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Carburetor Backfire on acceleration at low speed

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donnercm

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Hello,

I have been having a problem with my 100-6 back firing on acceleration with HD6 carbs. I have rebuilt the carbs, checked the ignition timing, changed heat ranges on the plugs and its still there. I recently check the compression and i found i have a couple cylinders down.

#1 - 140psi
#2 - 100psi
#3 - 140
#4 - 135
#5 - 120
#6 - 145

What is an acceptable variance btw cylinders?

Thanks,

Chad
 
donnercm said:
Hello,

I have been having a problem with my 100-6 back firing on acceleration with HD6 carbs. I have rebuilt the carbs, checked the ignition timing, changed heat ranges on the plugs and its still there. I recently check the compression and i found i have a couple cylinders down.

#1 - 140psi
#2 - 100psi
#3 - 140
#4 - 135
#5 - 120
#6 - 145

What is an acceptable variance btw cylinders?

Thanks,

Chad
Click to expand...
Look for a vacuum leak at the intake manifold area. I had the similar engine problem and had a cracked intake manifold by #6 cylinder. I'm overhauling my engine after #6 piston broke apart so I don't know if it was 100% of the cause of the back-firing or not.
 
Hi Chad,
Are you talking about a backfire through the exhaust or a backfire through the carbs?

Keith
 
Through the carbs. I checked for vacuum leaks, i found a large one where the manifold drain plug was missing on the rear of the manifold. I plugged it and the car idles better, but when i rev the engine i am still getting popping out of the carb.
 
Sounds like a very lean mixture. I would be concerned about the 100 psi on #2. That is rather low, you could have a burned or improperly seated valve. Check the valve clearance on #2 and all the cylinders while you're at it.

I would look at the fuel levels for each SU. You can check this fairly easily by removing the piston/damper units. On my 100, if you pull the choke to lower the main jets, you can see the fuel level in the throat of the SU, it is supposed to be about 1/8 in below the top of the throat. Don't know if that procedure applies for the 100-6 SU's, but I think it does. Set the unchoked jet height in the two carbs so they are similar to each other.

If you can't find any more vacuum leaks and the carbs are set up similarly, that low compression in #2 could be the problem. If an intake valve is leaking, you will have low vacuum and a lean mixture. Fixing this will require a valve job, I suggest you try the easy stuff first.

Good luck!
 
When you say "check the valve clearance" are you referring to the valve lash? I have checked all of the valves and the are set at .012" hot.
 
I had this same problem when I first started my engine after a rebuild. I eventually traced it to two problems. The first, and I think the most important, was a VERY low fuel level in one carb. So I agree with "Healey 100" on that. My timing was also off a bit, and I think this contributed to it.

Keith
 
Do i need to be concerned about the lower compression on cylinders 2&5? Could a blown head gasket cause the backfiring?
 
I think the low compression is probably caused by burned valves or excessive ring wear, but other more knowledgeable members may have a better idea. I would definitely be worried about the low compression, but I don't think it would cause the carburetor backfires. You might try a wet compression test (squirt a little oil in each cylinder before the test). If this brings up the compression, the problem is with the rings. Blown head gaskets usually result coolant in the oil or oil (or an exhaust smell) in the coolant in my experience.

I would check the gas level in each float bowl and try making the mixture richer in all carbs (until you get smoke out the exhaust) and see if that helps with the backfires. Also, double check your timing. This worked for me, and my symptoms (except for the compression readings) were very similar (popping at on acceleration).

Keith
 
I would suggest doing a leak-down test, that will tell you whether it is a valve leaking or rings. If you aren't familiar with the leak-down testers, it is a matter of pressurizing each cylinder with the piston at TDC of power stroke then measuring the leakage. Then you can listen for the hiss - if the hiss is in the exhaust pipe, the exhaust valve is leaking, if you hear it in the carb, the intake valve is leaking, if you hear it in the crankcase, rings are leaking. If you hear it in all three, time to rebuild the motor.

A little internet search will find several choices of leak-down testers, or maybe check your friends. Most auto repair shops have them and can do it for you.

#2 & 5 are low and will affect power and smoothness. The popping suggests to me a valve is the likely culprit but....

BTW, there is a drain at the front of the inlet maifold too you might want to check. Also, I am assuming you had the throttles blocked fully open when you did the compression test.
 
donnercm said:
When you say "check the valve clearance" are you referring to the valve lash? I have checked all of the valves and the are set at .012" hot.
Click to expand...
It'very important to set the valves correctly. Doing Hot is my preferred method.

Can someone chime in with the "13" rule(or something like that?). I use the EO-IC method,but not everybody does it that way.
 
Patrick67BJ8 said:
Can someone chime in with the "13" rule(or something like that?). I use the EO-IC method,but not everybody does it that way.
Click to expand...


Rotate the engine until a rocker is at 'rock;' i.e. the valve side of the rocker is all the way down (valve open). Subtract the number of that valve from 13, and adjust the valve with the number of the difference. Example: valve# 12 is at rock, subtract 12 from 13 (13-12) and adjust the lash for valve# 1. Rotate engine until valve# 11 is at rock, adjust lash for valve# 2, ...

For a 4-cyl it's the 'rule of 9.'

https://www.classiccar.co.nz/articles/how-to-adjust-tappets-and-valve-clearances-230
 
OK, so i did a leakdown test and it was definitely the exhaust valves on 2&5. Supposedly when i bought the car the engine was "Overhauled" and its never been driven since just run in the garage. I pulled the head and nothing seems obvious except it looks that the valves on 2&5 aren't seated all the way. Also it looks as if the valves are original based on pitting on the face of the valves, should i replace the exhaust valve with the unleaded upgraded valves? Is it necessary to replace the intake valves as well with unleaded valves? What about the valve seats, do i need to install inserts?

Sorry for all the questions, while i have it apart i want to do it right.

Is Dennis Welch Racing a good resource for parts or is there someone in the states that i can get good parts from?

Thanks,

Chad
 
donnercm said:
OK, so i did a leakdown test and it was definitely the exhaust valves on 2&5. Supposedly when i bought the car the engine was "Overhauled" and its never been driven since just run in the garage. I pulled the head and nothing seems obvious except it looks that the valves on 2&5 aren't seated all the way. Also it looks as if the valves are original based on pitting on the face of the valves, should i replace the exhaust valve with the unleaded upgraded valves? Is it necessary to replace the intake valves as well with unleaded valves? What about the valve seats, do i need to install inserts?

Sorry for all the questions, while i have it apart i want to do it right.

Is Dennis Welch Racing a good resource for parts or is there someone in the states that i can get good parts from?

Thanks,

Chad
Click to expand...

Chad,

Can you take a couple close-up pictures and post them for us to review? Particularly the valves that aren't seating, and the cylinder bores.

I imagine you'll get some more accurate advice that way.

I have a hunch that if the engine was freshened up, then maybe sat for a while, you could have some sticking valves? That could explain your backfiring and weird compression readings.

Thanks, Jeff
 
Patrick67BJ8 said:
donnercm said:
When you say "check the valve clearance" are you referring to the valve lash? I have checked all of the valves and the are set at .012" hot.
Click to expand...
It'very important to set the valves correctly. Doing Hot is my preferred method.

Can someone chime in with the "13" rule(or something like that?). I use the EO-IC method,but not everybody does it that way.
Click to expand...
Here's good reading on the EOIC method to adjust your valves. I've been doing it this way since 1975.

https://www.macysgarage.com/myweb6/valve_adjustment.htm

https://www.chevyhiperformance.com/techarticles/148_0303_setting_valves_info/
 
Jeff,

Yes that is the case, the car sat in mid restoration for about 10 years before i bought it and got it running.

Also, i took some pictures but i am a little unsure about how to upload them here.
 
donnercm said:
Jeff,

Yes that is the case, the car sat in mid restoration for about 10 years before i bought it and got it running.

Also, i took some pictures but i am a little unsure about how to upload them here.
Click to expand...
Posting pictures can be complex to do at first. To make them low res to post and upload I use Picasso-3(free), and resize them that way <span style="text-decoration: underline">first</span>. It gives me a new low res file and also keeps my original unmodified.

On Picaso, select he picture you want to post and then click on "Export" at the bottom of the Picaso screen and a option pops up with selections of what size you want to save the new file(I always go to the default lowest res one). Once that is done you have a new low res file to post.

Go to File Manager on BCF and then open and select the file/picture you want to post.
 
I got a free Photobucket.com account and upload photos to it. Then I copy the image file info and post to this forum. No resizing needed.
 
Chad:

I have had the best result taking my cylinder heads to a reputable, local automotive machine shop and ask for a valve job. They will check and plane the head flat, magnaflux for cracks, and grind all the valves and seats appropriately. If you need new valves, guides, springs, etc, they will let you know and let you supply them if necessary (Not many shops list Austin Healey parts from their usual suppliers).

Our shops here charge about $300 plus parts for this kind of work.

It is very difficult to inspect a cylinder head yourself and know that it is OK. I think it's best to leave the inspection and refurbishment of your cylinder head to a professional. They have seen thousands of heads and know exactly what to look for and how to test them.

Good luck!
 
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