40 Backfires only between shifts?

[Blue Ox]

My 1978 40 has developed a bad backfire, but only on engine deceleration between shifts. No backfire on deceleration downhills. I'm running out of options to fix it.

Background:
The truck idle was getting rough and stinky exhaust, so it was time for a tuneup. No backfire at any time. Stock Aisin carb with all of the emissions stuff. It passes the smog-enhanced requirements in Kalifornia
Timing set at the BB.
Valves checked and adjusted
Lean drop idle mixture adjusted, 650rpm, 20" vacuum at idle
plugs cleaned and gapped
fuel cutoff solenoid has 12v and appears to work properly
No exhaust leaks
Possible problem: fast idle screw has fallen out, but without it, fast idle sits at 1400rpm

After tuneup, engine purrs, idle is smooth, and revving in garage produces no embarrassing sounds. When I drive it on flat roads, slight burble in exhaust pipe between shifts. When I push it up hill, it makes a loud backfire between each shift, Driving downhill in second and letting engine compression slow the truck produces no backfiring at all.

Since this situation only started since I did the tuneup, I suspect it is carb related. ABV is all hooked up but I haven't had to check it in the past since the problem just occurred.

This problem has been discussed in the past, but the OP didn't post what the problem was when it was solved.

Any ideas?

 

[LAMBCRUSHER]

if the air bypass valve doesn't close right, you could be getting air into the exhaust tract without having a leak.....might check that out...

 

[Blue Ox]

Thanks for the idea. If the ABV doesn't close correctly, is it possible to make a fix? I don't see any way to dis-assemble it. What If I shoot PB Blaster into the two hose ends (from the air pump and to the manifold) and a squirt into the ABV where the small hose attaches. Can this loosen a stuck or make the problem worse?

 

[LAMBCRUSHER]

don't know how that stuff will work with the diaphrams...there is an FSM test procedure floating around here somewhere, find it, follow it and go from there....where did you say your ignition timing was set?

 

[Blue Ox]

I found the ABV test procedure in the FSM for emissions, but haven't been able to determine if it works yet. Perhaps a soap solution applied at the air relief opening will tell.
I agree that the PB Blaster might not be good for the diaphragm, so I'll hold off until I am more desperate.
Thanks again for the ideas.

 

[Blue Ox]

Timing set at the BB. 7 degrees BTDC. When I first looked at it before I reset it, it was about 3 BTDC.

 

[LAMBCRUSHER]

if idle speed screw is in too far, fuel will be able to flow bypassing the idle circuit, which could allow for fuel delivery even during decel...try unplugging idle solenoid while running and seeing if it'll stay on...if it does, turn the idle speed screw out till it dies, plug solenoid back in and see where your idle is at. Did you plug the Vac going to the dizzy before checking time? How far out is the idle mix screw? 20 inches is alot, prolly no vac leak, but check by running at idle and choking out the airhorn with a piece of cardboard- don't use your hand in case you get a backfire...if idle increases after you choke it before it dies, you have intake leak somehow. if it just dies when choked out, you have no vac leak...HTH

 

[Pin_Head]

You need to adjust the throttle positioner screw to give 1,500 RPM and make sure it is working. It should have no vacuum at speeds above 15 mph and vacuum below that. If it isn't working, check the electrical connector at the smog "computer".

 

[Michael Hanson]

Hi, Got rid of our back fire when I got rid of the Magna Flow exhaust.and went back to Toyota system. Mike

 

[Blue Ox]

Thanks Lamb, Pinhead, and Mike,
I did the cardboard over the carb and it stalled immediately, so a vacuum leak is unlikely. I'll check the suggestions pinhead made and report back.
When I find a solution, I'll make sure to post it!

 

[Blue Ox]

OK, I think I'm making progress.

I removed the hose from the ABV to the check valve at the manifold while the 40 was idling. Lots of air pumping through the ABV at idle.
I plugged the hose to the check valve with a cork from an outstanding ale (Sierra Nevada 30th anniversary barleywine ale).
Test drive yields no backfiring at any time and perfect running.
The ABV appears to be be leaking, although I don't know if it is caused by a problem with the ABV itself, the computer or the VSV.
At this point I will drive it it with the cork in it and hope it doesn't cause any other problems. I am going to shoot some WD 40 (less caustic than PB Blaster) into the ABV and periodically check to see if it has unstuck itself (please), and start looking for a ABV to buy and replace what I have. Anyone have one that you would like to send to a good home?

Thanks again for everyone's help and suggestions. I would have given up on the 40 years ago had it not been for the fine pool of experts who share their expertise on this site!

 

[Pin_Head]

What makes you think the ABV is bad? It is supposed to send air to the air rail except at high manifold vacuum situations.

 

[Blue Ox]

Good question. I need to watch the airflow out of the ABV (disconnected from the check valve) and have someone rev the engine and then let the revs drop to create the high vacuum and see if the air flow out of the ABV turns off. If it does, the ABV is probably OK. If it doesn't then the ABV may have a problem.
Either way, when I disconnected the hose from the ABV to the check valve and then plugged the line into the check valve, it completely eliminated the backfire.
Am I thinking about this wrong?

 

[Blue Ox]

I ran the 40 at idle. low volume, but good pressure air coming out of ABV. Increased rpm about 1000rpm and flow is high and strong (I don't have a way to measure air pressure out of the pump. The air flw coming out of the ABV wouldn't blow up a balloon, but if it was blowing into your ear, it wouldn't be romantic.
Revved the engine and let it drop and air flow continually came out of the ABV, high volume at high rpm and lower volume falling and low rpm.

Can I test the ABV by sucking on the hose to the ABV at the VSV end? I suppose there could be a problem at the vsv or the computer to cause this and I'd like to isolate where the problem is.

 

[LAMBCRUSHER]

does the small line going to the ABV have full vac? I believe the valve closes with a high vacuum signal...

 

[LAMBCRUSHER]

also check for 12 volts at the connector to the VSV(switching valve)

 

[Pin_Head]

The ABV shuts off air during very high intake vacuum, like engine braking going down hill. Since it isn't backfiring under this circumstance, it is probably OK. The throttle positioner prevents popping in the exhaust when the ABV is open supplying air to the exhaust. It works by supplying a rich enough fuel mix to ignite so it is burned by the time it hits the hot exhaust manifold with air in it. Just removing the air injection system will fix the problem, but it won't pass smog check in CA.

 

[Blue Ox]

Lamb and Pinhead,
I really appreciate your suggestions. I'll check each of them out and report back. I'd like this 40 to run well AND pass emissions, so I'm not out of the woods yet.
Thanks again for directing my attention and efforts. I'll be back under the hood after work today. If we can find the source of this problem, the solution may also fix my nearly too high CO problem at the smog station.

 

[Blue Ox]

OK, I think the problem is solved. Lambcrusher and Pinhead both sent me in the right directions and the 40 now runs beautifully with the barleywine cork removed and the hoses from the air pump through the ABV all hooked up correctly. Excellent engine response and idle and no backfire of any kind on deceleration or shifting.

Solution:
1. Found no vacuum in the line from the VSV to the ABV. In addition, the line from the VSV to the BVSV blue had 18 in. vacuum. No vacuum at ABV meant no diaphragm actuation to redulce air flow to air intake manifold upon deceleration. I think this caused the original problem, although I don't know why this has never been a problem in the last five years I have owned the truck.
2. Several other ports on the VSV were connected incorrectly. I recorded and drew what the original setup looked like.
3. I replaced the missing throttle positioner screw. Removed the vacuum hose from the TP diaphragm and noted movement indidating it still works. Reset the TP screw per FS emissions manual.
4. Removed the hoses from the VSV and re-routed all to the FSM diagram.
5. Removed the barleywine cork from the hose to the AI manifold, reconnected this hose to the ABV air output side, and put air cleaner back on.
6. Test drive the truck through the local hills and it drives and sounds wonderful! Tomorrow I'll take it to the smog station and test the HC and CO and see if the CO has come down!

I hope this is useful to someone else who encounters this problem. I truly appreciate the attention and guidance given by Lambcrusher and Pinhead.

 

[sloanstavern]

Excellent thread. I was in similar boat, just adjusted valves and timing and the world went to pot. Rather than re check my work for the third time saw this post and redirected my efforts . Found this broken link from the intake. My last 40 was smog free so this is all new to me