I started my 235 up for the first time on Saturday. It is a 59 truck engine bored .060, fresh ground crank, rebuilt 848 head. Dual Fenton exhaust and Intake. I bought two Weber 32/36 from Tom Langdon. It has HEI ignition. It started up fairly easily and ran smooth. I adjusted the timing to 12 degrees advanced at 500 rpm. It was retarded at first, but I corrected that once I got my timing light on it. I set the idle mixture screw after turning it in and the engine struggled, and I gave it 1/4 turn out on both carbs. Looking at YouTube videos on tuning these carbs I have a few questions. So far, I like the versatility of these carbs and their ability to tune more precisely. I liken them to a VM Mikuni on a snowmobile. I'm good at those. My questions are, The idle mixing screws are at 1 turn and the primary venturi jets are 75. Too big, Too rich? I need smaller primary jets. How small? SECOND question is the secondary venturi jet is 60. Too small? Too lean? I'm getting a sputter/backfire when I get into the secondaries. It does not happen when not loaded, just when driving. I've tried different timing but its not a fix. I even tried to not have vacuum advance as the HEI does not require that I use the vacuum advance supposedly. Without the advance I get about 25 degrees at 2500 rpm. With the vacuum advance I get 45degrees at 2500. Both start at 12-15 degrees at idle. I'm thinking the jets are too small on the secondaries and I am lean, causing the backfire.
Thank you for the vote of confidence Fibonachu, but these are NOT my "cup of tea".
My experiences with Webers on factory installations has been great.
But my experiences with Webers on non-factory installations, would have to been better than they were, to be classified as horrible.
Would suggest contacting Mr. Langdon.
Good carburetion is fuelish hot air The most expensive carburetor is the wrong one you attempt to modify If you truly believe "one size fits all" try walking a mile in your spouse's shoes! [image]http://www.thecarburetorshop.com/Avatar.jpg[/image]
Purchase an AIR / FUEL ratio gauge, it will save you a ton of hours of tuning. About 5 miles of driving will tell you everything you need to know about jetting. Leave the timing as it was before the carb install. On sale this weekend, https://www.holley.com/products/clearance_parts/parts/4651
Does your HEI ignition scheme include anything made by Pertronix? It looks like you have a stock distributor and vac advance. 12 to 15 BTC at idle with no vacuum sounds high as a starting point. I'd drop that to between 7 and 10 BTC. I have HEI (Tom Langdon's unit) and something around 8 has worked for me. Electricity invariably follows the path of least resistance. If you're too far advanced spark may leap to the distributor post adjacent to the one it is supposed to hit and that will cause problems.
My HEI is from Deve's Technical Network. It uses the original distributor. I'll back down the timing and try that. It runs very well at idle and not moving truck, when you get on it with a load it acts up. Thanks.
Jon H, the engine is a new rebuild so timing was never set. Jon G, I backed the timing down to 8 degrees, It is better but not perfect. If I drive gently, it behaves and does not act up. If I drive aggressively and floor it, it will spit, sputter, and backfire though the rear carb, never the front carb. Thats the odd thing. I had it up to 55mph by slowly getting there. I have a Gear Vender Overdrive behind my SM420 and the 4.56:1 rear end ratio. 50mph is about 2000rpm. Final ratio should be 3.55:1. The truck has plenty of power and water temp steady at 185 degrees, oil pressure at 35lbs. Trany shifts fine and is smooth. The rear end, I put in new wheel seals and pinion seal. Brakes are the Huck brakes, all rebuilt and work well. This is a 47 3600 Chevy. The air fuel ratio gauge would be the best for tuning, I'm sure. I put EGT on my snowmobile to get the tunning right on, so I know it would be the best way to dial these carbs in. I think the primary jet is too large because my mixture screw is at 1 turn.
If you're having trouble from hitting the gas when the truck is loaded it says one thing for certain...by doing that you're driving the manifold vacuum down low. Probably into single digits. That will naturally drop out the timing advance provided by the vacuum biscuit but there's something else at play. I think timing is a factor but maybe not the only one. Let's consider if you had a Rochester with a working power circuit (rare, sadly) or Carter YF with a working power circuit (less rare) dropping that vacuum suddenly would cause your air/fuel mixture to go almost instantly rich. Very rich. In those two carburetors it happens this way: there's a spring that constantly is in play on the power piston in the Rochester and on the diaphragm (and coincidentally the metering rod) in the Carter. Vacuum pulls against the power piston or the diaphragm and that keeps the power piston from opening the power valve in the Rochester and it pulls on the bottom side of the diaphragm in the Carter which keeps the metering rod in its lower range (the leanest range) in the Carter. Deny either of those carburetors vacuum and you get a much richer a/f mix instantly. Why? That's how they're designed. That is so your engine gets the fuel it now thinks it needs to pass another vehicle, climb a hill or move a heavy load. Now in the Webers you've bought I've never studied their power scheme, however as I recall it works with a diaphragm (vacuum controlled) and a regulating spring. My guess is your first problem is running the engine a bit too far advanced. Since I don't know for sure I'm going to assume your vacuum advance goes to ported vacuum and not straight to manifold vacuum. If I'm right at idle if you disconnect the hose going to your vacuum advance biscuit there will be no change in timing...ported vacuum won't provide (or should not provide) any vacuum to that biscuit. And here I'll add it is important to set your timing at idle using the factory specified idle speed...because remember speed and rotation (centrifugal force) are what make your mechanical advance operate. Run your idle up too high or down too low and your timing will not be what the people who designed your engine wanted. Even 100 rpm one way or the other will cause a difference. Anyhow back to your carburetors, I would start studying them to see what makes their power circuit tick. If you're getting backfiring only out of one, then there is a reason. Something is causing the front carburetor to work differently than the rear one (obviously). Please remember too lean of a mixture and too rich of a mixture can cause backfiring. Too lean? It will burn more slowly...possibly too slowly. Too rich? All of it may not burn before the exhaust valve opens. Either way unburned gas goes where you don't want it and can ignite. In my experience backfiring through the carburetor is nearly always caused by an overly lean condition. I've even seen this occur as the result of a bad accelerator pump and since you have 2 carburetors this might be your answer...one pump is okay and one is not working. You hit the gas and one of the accelerator pumps doesn't shove any gas into the manifold...just a bunch of air and what gas normally follows through the venturi and then "bang" goes the backfire because the mixture is wrong. Good luck. Hope some of this helps.
The vacuum is ported and at idle it is non existent. I left it off the distributor and plugged the line to the carbs. I then only get the mechanical advance and whatever the HEI has built into it. It is better but did not eliminate the the popping back in the rear carb. I talked to Tom Langdon and he said to check a few things. I completely disassembled all the jets and checked them. He feels the popping back is lean and could be a plugged jet, but I have not found that in the two times I’ve had it apart. I’ll keep trying different things. I enjoy the challenge. I do have a heated manifold and it gets pretty warm so I don’t think it is related to a hesitation problem. It is quite responsive to the throttle and really doesn’t have a dead spot. I can try some more timing adjustments tomorrow. Maybe I should check out the front carb and see how it is set up? It seems to work fine. I could switch them places and see if it fallows the carb. I’m going to check my compression too, now that I have a few hours run time on it. Could possibly be a leaky valve on my new head? Although it sounds good and runs smooth.
Yes, switch the carburetors and see what happens. Also is your vacuum steady, wandering through 3 or 4 pound sweeps or jittery? A compression test is a very good idea. Good to put your mind at ease. Check also both accelerator pumps. Good on the ported vacuum. Personally I'd hook vacuum back up to the distributor. I know there are folks who'll say you don't need a vacuum advance with HEI, but I'm not in that group. Very lean condition in addition to burning slowly can burn unevenly and that will also cause backfiring as rpms increase (won't all burn before the next intake stroke occurs). Good luck!
I GOT IT! I GOT IT! I reset the timing by the old 59 235 spec, reset idle speed to a little less than 500rpm. I have a analog tach, so, the manual calls for 475rpm. I would have to get a digital tach to be more accurate. Re-attached the vacuum advance to the distributor, cleaned carbs and checked jetting. It runs PERFECT! Lots of power and no spitting or backfire. Thanks for the suggestions and help. Compression is 135- 140 on all cylinders by my Matco gauge.
Last edited by lumbersawyer; Sat May 27 2023 09:10 PM.