66 C10 Carburetor Advice for 292 engine w/307 rear

[1966Step]

My 66 C10 has a 63 292 (3 on tree) that came out of a school bus (installed back in 1981). Long history and haven't driven it much until I swapped the rear end from a 373 to 307 last year. I have a Rochestor B single barrel (that came off the original 230 I think) and I am looking for advice on a better combination. My truck is slowly turning into a daily driver and I am looking for a combination of good power and fuel economy.. I'm also wondering if the air cleaner assembly is correct since that also came off the orioginal 230 (I think). She runs pretty well but I'm wondering if this setup is ideal. Thanks in advance for any feedback!

[Wrenchbender Ret]

The 292 engines had a carb with a larger bore then the 230-250 engines. Most of the time the better carb is gone, replaced with one of the smaller bore carbs. You need the larger carb. but they are hard to find. It doesn't look like you have HEI. The HEI from a 75 up 6 cyl. will fit & make it run better. You will want to change the advance mechanism to get faster, & more advance. The exhaust system on 292 engines are very restrictive. You should use a split manifold or headers or at least larger pipes & less restrictive muffler.
George

[1966Step]

Thanks so much for the info George! I will look into HEI. Just got the truck back from having the carb rebuilt and then started backfiring occasionally (7-8 times) on my first trip to VA (over Potomac) in 34 years. (3 hours). Idles fine but a fair amount of bucking when I barely have the gas on. THEN, wouldn’t start (usually goes on first turn) when I was leaving work Thurs and finally started and backfired like crazy and blew out my muffler ☹ . Limped her home and just started her today and seemed OK, So gotta fix the problem before a new muffler but thinking about going dual since I have to replace it anyways. It’s just $

Here’s a pict in front of my house. My GF wants me to work more on the house.. jeez

[MikeB]

Maybe some help here:

https://www.jalopyjournal.com/forum/...advice.244017/

http://67-72chevytrucks.com/vboard/s...d.php?t=775927

[1966Step]

Thanks so much Mike. Both links are great resources and looks like I have more research to understand my best options. I can do easy fixes on this truck but this is getting advanced for me. I never really had a backfiring problem until this carb rebuild and so I am trying to understand everything before I take the truck back to the shop with the blown muffler and see what they say (also regarding the new problem of not running very smooth when just cruising).
I have recently purchased a vacuum gauge and timing light (arrived yesterday) so I can learn to use those and become better educated when talking to the shop. My goal for today is to find best spot to hook up the vacuum gauge and see what I can find out there.

Here’s my blown muffler. I know it didn’t have much left in it. I’m wondering if all my efforts to start the truck made it backfire like crazy (20 times at least). as I had her running for 30 minutes yesterday and she was purring pretty well I thought. For the 40+ years I’ve owned the truck (sat for many of those), I never had a backfiring problem.

Thanks again for your helpful links!

[1966Step]

OK, Got my Vacuum guage hooked up to a port of the intake manifold and got er warmed up. Reading 14-15 with old filter and 15-16 with new filter. Definitely seemed to get more punch which was expected out of the new filter when I hit gas and it went down and then into the 20s which I think is normal. Now doing research on my readings because they seem low.

Also, found a great Youtube video for different carburetor exhaust setups on a 292 with a graph showing the HP and Torque effects of stock 1 barrel, stock with a 2 barrel, Clifford & Headers with a 2 and 4 barrell which was super helpful!

https://www.youtube.com/watch?v=Tt2UxQCwAJo

[MikeB]

Could be the 15-16" vacuum is due to normal wear and tear on rings or maybe a slight leak around a valve seat.

Have you played with the idle mixture screw to see how it affect idle vacuum?

[1966Step]

I just tried adjusting the idle mixture screw (and waiting) and it was having little if any effect on the vacuum. Then it started slowly declining past 10 and I just turned it off and came in to seek warmth for a bit.

[oem4me]

Look into ignition issues. Years ago, I had a frustrating backfiring problem that eluded me for way too long. As it turned out, an undetected nick in the wire that goes from the negative side of the coil to distributor was burning out distributor condensers and wreaking havoc with ignition. Initially, I found replacing the condenser would solve the trouble ... for awhile, then it would start missing/backfiring again. I finally discovered the partially cut wire, fixed it, and never had another misfire or blew out any more mufflers.

[1966Step]

Thanks Oem4me! I will check that out once I get some sun and warmth today. Also, I am going to try to advance the timing a bit as it is also heating up which I've found is also a symptom of retarted timing. The shop I took it to rebuilt the carb but didn't change out a completely blocked air filter (That's my bad for having it get to that condition.). Now I'm wondering if they even messed with the timing which hasn't been touched in years, but was running fine before they "fixed" it.

Didn't drive the truck much until I swapped the 373 for a 307 last Feb and I've put 3300 miles on her since. The last time I put that many miles on her in a year was 1985 and Ronald Reagan was the president

[grubsster]

I agree with Wrenchbender, especially regarding the HEI. As oem4me has eluded to, I have had so many backfire issues caused by something ignition related. Bad plug wires, a crack in the distributor cap etc... I don't know when your last tune up was but, check the plug wires are going to the right place. Try to see if you can see arcing anywhere. Good luck and that's a nice looking truck.

[1966Step]

Thanks Grubster! My wires, plugs, Dist. cap etc.. are about 2 years old. But, just pulled plug 1 below and it is black and looks pretty bad (and so was 2) One cause of the soot among many others can be a dirty air filter which I was very guilty of. Gonna run and grab some new plugs and take that variable out of the equation for now.
I also checked the wire from the coil to the distributer and it’s pretty old so I’ll replace that too. Thanks again!

[grubsster]

Those plugs look pretty rough. I would replace all the plug wires and verify firing order. 1-5-3-6-2-4. Just my .02

[MikeB]

The black plug indicates either oil or a way too rich carb. But I don't think either would cause backfiring. Regardless of which plugs are in there now, be sure to buy the correct ones for your engine -- heat range and thread length.

[1966Step]

I have e3.46 plugs which someone recommended when I replaced 2-3 years ago. Same specs but it has a fixed gap and can’t be adjusted. They’ve done me good until this latest episode. Will probably change to ACDelco’s or similar and get some new wires. The firing order is correct.

I switched plugs today and leaned up on the fuel mixture by ½ turn. Managed to make it 1 mile to the gas station and still running rough and backfired once but seemed better. I will mess with the timing this week as I just bought a light and need to learn more on how to use it. Tonight it’s Youtube on that subject and football.

Also, I’ve mostly been taking her on short ½ hr backroad drives (up to 50 MPH to work) and such since I had the rear-end changed to 307 and a lot more short 10 minute drives where I live. I saw that short drives and idling can cause carbon deposits on the plugs. So my recent trip to VA (3 hours) was something my truck hasn’t done in 30+ years!

Thanks everyone for chiming in with your thoughts. I appreciate it!

[1966Step]

Well after a new fuel pump, plugs, wires, points, rotor cap, distributor cap and hours of frustration. I discovered someone must have reversed my firing order at work the day the trouble began. I checked it initially and inadvertently counted counterclockwise. This was never in the equation as I'm super careful when messing with those wires and I wouldn't think it would even run in reverse order, but made it for a 30 min ride home that day. Unsuprisingly, she's running great now but need to change the oil again as smelling gas in it after a couple of hours troubleshooting while the order was still backwards and just did an oil change last week just for that reason. Feeling dumb, relieved, and pissed off all at the same time!

[MikeB]

That's good news! At least you didn't spend a ton of money and ended up with several new parts.

[1966Step]

Yep. And I learned alot along the way. That 292 has been so reliable (came out of a school bus about 40 years ago and still barely uses any oil). I haven't had to deal with all of those crazy problems I was experiencing.

[vince1]

Make sure the fuel cap is not pressurizing the tank. Some of the caps they sell are wrong and don't vent both ways. With outside air temperature changes my crankcase would fill with gas after the truck had sat for a long time. Removing the cap would result in a huge rush of escaping air.