Projects Shade tree Model A speedster kind of thing

Check out this chart. Your carb might be too small for your displacement

 

I don't think the SU is undersized. Model A engines are 201 cubic inches (3.3 liters), but they turn so slow. I think that chart probably assumes much higher rpm's. If you look at the hp scale on the bottom of that chart, the single 1 3/4" should work for 50-100 hp, which is in the ballpark. The stock carb flows around 100cfm, an SU HS6 flows around 210 cfm. If anything, it should be too much carb, but I don't think that's as critical with this design because of the variable venturi. You guys feel free to keep the suggestions coming, though. It's probably something stupid I overlooked.

 

OK. I'm officially stumped. I took the carb apart, checked the float height, blew out the jet (old SU's only have one) checked that the needle was centered, it still is so lean it will barely stay running with the "choke" pulled all the way (these don't have a choke, either. There is a mechanism to drop the jet body down on the needle, giving a rich mixture). I am confident I don't have a vacuum leak. The only thing that has changed since i swapped carburetors is the exhaust. Every step of the way on this exhaust project, the car has run worse and worse. With the stock exhaust and the SU swap, it ran well, better than with the stock Zenith. Dual exhaust with the elbows running under the frame it would run with the choke out. When I changed it so the elbows went over the frame it ran with the choke out, but backfired and hesitated. Current side pipe deal it will hardly stay running fully choked, hardly take any throttle, backfires even worse. Any ideas out there? Haven't messed with the timing during all of this. Same gas the whole time.

 

Maybe try slapping the stock exhaust back on to see if that cures the current problem just to eliminate any other possibilities for sure.

 

I'm going to look for another stock exhaust manifold and try that. I cut up the two I had. Haha.

 

If it proves to be a back-pressure issue I suppose you will have to baffle whatever you decide on to achieve what's required to get the performance balanced, and baffles are a design entity all on their own...all part of the fun...
@55Deso's suggestion is a good one. Re-verify to isolate that as the issue.

 

Well, Fellas, I kind of out-smarted myself. Turns out part of my problem was that the coil has been failing. Tricked me because it has been keeping time with my exhaust project. I replaced it and now its running as well as it did with the first dual exhaust set-up, so I'm at least back where I started. Still backfiring on decelleration even at the maximum rich setting. I do have some upstream exhaust leaks, though. It takes gas well, but stumbles if I try to hold a medium fast rpm. Haven't had a chance to road test it yet, all this Alberto rain is getting in the way. It sure is loud. I think baffles are still in order.

 

You can try these, they worked on my motorcycle that was having the same problem with straight pipes and no mufflers.



https://smile.amazon.com/Long-Exhau...d=1527546368&sr=8-18&keywords=baffles+exhaust

 

Enrich the idle mixture. That is the only circuit working when the throttle is closed. I'll just about guarantee it to stop the popping. Learned it on a mini-bike in 1968 and have used it successfully on innumerable engines since.

 

Awesome thread and build. Just read the whole thing. Good luck on the dirt roads.

 

Fabber,
SU 's don't have an idle circuit, as such. They just have the one needle that does everything. It's attached to a big piston that rises and falls by vacuum. At idle, it almost completely blocks the venturi. Open the throttle, the piston rises, pulling the needle out of the jet and simultaneously opening up the venturi. Basically, the piston controls the amount fuel and varies the size of the venturi, where conventional carbs have a constant venturi size. I have been fooling around with them for years on old british cars and Volvos, and they have really worked well. The profile of the needle is kind of critical, and picking the right one apparently relies on some kind of magic, but they are pretty forgiving, all things considered.

 

I'm slowly chipping away at this lean mixture problem. Extended the exhaust with the flared front section of a 1940 torque tube I got off of the classifieds here. My original plan was to use it for the side pipe, but I didn't realize that the walls at the front end of those were almost 3/8" thick. It weighed about 20 pounds. I cut it back to where it matched the diameter of the driveshaft pipe I made and welded them together. At least it extends a bit past the passenger seat now. Those backfires were going to be exciting with the shorter rig. I also fitted an air cleaner, which hit my hood, so I cut a semi-circle out of that, then realized that rain was just going to drip straight into the carb, so I used a bit more of my salvaged aluminum and made a baseball cap brim for it. Its riveted on. I still have to seal the junction between the aluminum and the wood, but you get the idea. Here are some pictures:



After taking a little more care to snug up the manifold/exhaust pipe connections, adding the length to the side pipe, and adding the air cleaner, I was a lot less lean on the mixture than before. Still needed to be richer than the mixture adjustment allowed. The hard core SU boys are probably going to yelp about this and start talking needle profiles and what-not, and they'd be right, but as an expedient, I just cut a 1/4" section of fuel hose and slipped it over my jet to prop it down (choke it) a little. You can barely see it in these pictures, just above the silver collar that's just above the tee that the fuel line attaches to. Anyway, it will idle and take gas now, and doesn't have the backfire issue any more, but it still needs work. I guess a baffle will be the next step. I'm going to order some 45 degree exhaust bends and weld them in place of the flex pipe so I can get it more solid and drop that side pipe down a bit. Thank you to anybody that checks this out.

 

All the posts have been well worth checking out. Old school use what ya got or can scrounge easily and inexpensively hot rodding creativity at it's best!

Ed

 

As you can see from this video, she smokes a good bit. #2 and especially #3 have oil on the plugs. I'm guessing worn valve guides, but I welcome a more experienced diagnosis. Compression test shows even compression across all four cylinders, about 60#, dry and wet. My tester is one of those with the little rubber tit that you press in, so there's that. Still, better than nothing. If you notice in the video, it seems to smoke most in high vacuum situations, which makes me suspect guides rather than rings, although I've heard the oil rings on these can be worn completely smooth without affecting the wet compression. Also, if that intermittent skip at steady rpm isn't a fuel delivery problem, couldn't it be from oil getting sucked in past the intake valve guide? At least it's running again with the SU carburetor. Anyway, any insight or theorizing is welcome. This along with the clicking from #4's exhaust valve makes me think a valve job is in my future.

 

Started work on the baffle. My first plan was to follow advice I found on here and cut apart an old glasspack. The sight of all that fiberglass gave me pause. Phooey on that. Took about 18" of old chain link fence tubing and drilled a bunch of 3/8" or so holes in it. (I just grabbed a drill bit. This aint precision work.) Took a leftover section of brake rod from the hood prop project and bent the shoulders of each hole. Hacksawed/ground/sanded/filed out a circle with some ears on it and brazed them together, bent the ears over and popped it in. I'll eventually drill it and secure it with screws once I hear it, decide if that's the right number or size of holes, see if I need to wrap it with fiberglass, see if I need to drill some holes in the periphery of that end cover... Anyway, see that blue tarp? It started pouring right after I took that last picture, and I don't even know what the damn thing sounds like. Good thing I'm patient. I'll let you guys know, maybe even take a smokey before and after video tomorrow.

 

As luck would have it, we got a little break in the rain just after I posted that last one, so of course I dashed out and started her up. It sure is quieter, and I was able to take the stop-gap fuel hose mixture enriching doohickey off of my jet mechanism. I'm back to the normal full rich adjustment, but it seems to like that ok. I'll know more when I can get out on the road. I think might drill some smaller holes around the big one in the end plate. Seems like a waste to go to dual exhaust and then just choke it back down to 1 1/2", or whatever that pipe is. If I go too far, I can always tap a couple of them and plug them up with bolts. I was going to do a before/after type thing, take the baffle out, record that, pop it back in, record that, but the rain interfered. Figured I had better just see what I had. Here you go:

 

I had Triumph motorcycles for
many years. Some of the carbs had slides with pins in them. The pins could be tuned by changing the position of a little C clip on the pin that would raise or lower
It. Don't know if this is something to look at.


Sent from my iPhone using H.A.M.B.

 

Here it is with some holes drilled. I center punched, but one of them still wandered on me. Oh well.

 

Nothing wrong with that. Just a thought, Does that carb have a vacuum advance port? Is it plugged?

 

Good thinking. It does have a port, but it is plugged. Everything I have done has helped fix the problem. What I think happened is that that the combination of a bigger carb, less restrictive exhaust, and no air filter gave me the lean mixture. Adding the air filter and increasing the back pressure with the length and bends and baffle in the exhaust has gone a long way towards bringing it back into balance. If I ever get around to upgrading to a higher compression head, I bet it'll be just right.

 

You might still to attach a vacuum gauge on the intake and see what its pulling. Probably should be around 18+ inches at idle. It would speak to other internal conditions like worn guides leaking valves and worn rings. How much can a model A head be milled to bump compression? I took .040 off my head and .010 off the block to bump my cr to 8+ to 1 from 7.1 to 1 on my 230 cut in 6.

 

I'm not sure, but I don't think milling flatheads is all that effective. They have such a big combustion chamber. I'll just keep my eye out for a good used high comp head. The vac gauge is a good idea.

Spent some spare time making an exhaust hanger out of an old heavy shelf bracket and some copper line. It works better than I expected. Don't tell, but those are just modern self-tapping sheet metal screws threaded into the ends of the copper pipe. If it doesn't hold, I'll make another strap out of steel.

 

This car is made out of stuff and things, that most people would toss out or use as parts to make something “ better”

This thing is just awsome !! And the way you go about doing your mods and upgrades is great. I would love to tool around in this thing.
It’s a big boys go kart !!!

 

That baffle is just reducing the total square inches of exhaust opening, reducing exhaust gas flow. Glasspacks work by allowing high frequencies to exit out of the exhaust stream while not restricting gas flow.

A better idea is the spiral or auger baffles sold by Speedway. The Speedway baffles are cheep enough, if your pipe is big enough to accommodate one (3"). Or if it's close enough you could grind the baffle down to fit. Or I'm sure you could fabricate your own version of this easily enough. Again, the baffle works similar to a chambered muffler, by cancelling the high frequency sound waves while not restricting gas flow. They are very effective and sound great. I think it would be a nice improvement, worth your time.

 

Blues, I hear you, but I think I need the exhaust flow restricted some, at least with the intake set up I have. It seems to have a pretty good air in/exhaust out balance going now. One of those spiral doohickeys would probably sound better, but I'm pretty happy with the way it's running currently.
Vandenplas, thank you. I'm not destitute or anything like it, but my whole deal here is to use what I have and not spend anymore than absolutely necessary. That adds to the fun for me. I haven't kept detailed records on it, and I know little things can add up surprisingly, but I was trying to figure out how much this thing had cost me the other day, and for the life of me I couldn't get it above $2,000. That's not counting the value of the stuff I already had, or of my labor. It'll never be a show car, and I'm sure not trying to pass it off as a copy of any traditional style, but I have had a blast working with it, and I'm smiling at it a lot more than I'm cussing at it.

 

That's why we are all here. At least that's my excuse. Like minded addicts sharing their passion, success or failure.

 

LOVE this thread and how you’re building this!

 

Have to say I have been wanting to do this myself lately, then I stumble upon your thread here, Yep going to find the parts to make the chassis I have a roller and driver.

Keep up the good work man!

 

We had some old stainless pipe, must have been some kind of duct work because it was just tack-welded together. Anyway, I made a turn-down for my exhaust from part of it. It's funny how things work out sometimes. The enlargement for the coupling is a perfect friction fit into the 1940 torque tube. Go figure. I'm just trying to keep the rain out of the pipe. It's parallel to the frame, so the rake of the car has it heading downhill. I collected a little water in it after a couple of hard rains. Anyway, I'm thinking the straight down looks better. I think twisting it so that it poots out to the side has a little too much of a muscle car vibe. I'll put a couple of screws in it once I decide. Opinions?

 

I prefer it straight down...hope you do something about those flex tubes. They are F@#$%up a pretty nice build.