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syncing rear carb
Hi all. I am more than happy with the way my tuning is progressing. Thank you all for the input. Most of my problems ( you'll remember my lament about fuel spraying out the carb top and carbs not drawing) centered on float adjustment and carb balancing. This brings me to a question- on my Factory Five car the rear stack for the carb( 48IDA) is half under the cowl. I really can't get my syncrometer under that space to fit into the stack of the rear carb. Any solutions?? Jon
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make an adapter from a radiator hose and use that setup on all? Or take off all the horns and sync them that way.
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Tom, I thought about constructing a pvc plumbing fitting with some foam seals to bridge the angle. I like your hose suggestion and confirmation that I need to do all the carbs with it. I look at pictures of other cars and they just don't show the conflict. Shows you how much these cars differ by manufacturer. Thanks Jon
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Take some vacuum hose , couple of feet, Stick one end AT your ear, not in your ear, and the other end at the top of each stack.....use it like a stethoscope and listen to the hiss....when they are the same...then they will be perfect...
Try adjusting the screw in and out so that you can familiarize yourself with the variance in sound....... |
the video i watched you need to do the 4 center stacks,thats how i do mine
get the 2 on one side to pull the same #'s and then the other side to pull the same #'s could be 5 on one side and 7on the other then i balance the two sides with the center rod and set idle.the other 4 should all be the same. |
Carb Synchronizer VW Bug Jaguar Healey MG Triumph Weber IDF Dune Buggy Unisyn | eBay
These allow more precise adjustments. |
Rick, you've been a great help with this tuning. I was about to buy one of those unisync devices to fit in the space better- actually had one years ago for some SU carb tuning- but some on line reading suggested they were not the tool to use. They required a set restrictive reading on the carb. Maybe this was just the German manufacture trying to dis their competition. Regards, Jon
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Quote:
so for all practical purposes, just measuring the middle four venturies is sufficient. And that's the method prescribed by Jim Inglese as well. Z. |
The mechanical connection between the carbs ....ie ..the linkages should be adjusted before you start the engine, starting at thhe throttle plate closest to the main idle screw and then adjusting the others to be "mechanically" in sync. You cannot adjust mixtures unless the carbs are mechanically perfectly linked....first...
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Quote:
You should not have to place the synchrometer on the 2 throats under your cowl. Take a reading on the carb connected to the main linkage. The second throat on that carb is controlled by the throttle shaft so you don't need to adjust that one. Adjust the adjacent carb via the attached throttle lever to match. The back throat (the one under the cowl) will follow via the throttle shaft so you should not have to adjust the air flow on it. Follow the linkage and adjust the 3rd carb, then the one attached to that one via the throttle lever. You really only need to adjust the air flow on 4 throats as zrayr says. If the attached throats don't match up you probably have a problem with the throttle shafts binding. John |
I have found that sometimes you have a 48 IDA Weber with different synchrometer readings on each of the two throats.
You can get that fixed by slightly twisting the throttle shaft such as to achieve perfect matching vacuum rates. |
And most of the time, a throttle shaft gets twisted from the throttle lever retaining nuts being tightened without any thought as to where the torque load is going.
I'll reread this thread after I add this. I'll have a guess and say that the rear barrel flows more than the front (LH bank; and the opposite for the RH bank) and I'd bet in most instances, this is the case when there is different flow within the same carb. |
I find this thread very interesting. My rear left carb is different by 1 number (5-6). I built a flow bench to dial in the carb by twisting the shaft when not on the engine. When I flowed the carb I had the same reading on both throats. So that means the intake manifold flows different, or the valves, or the piston rings.
Would then the best thing to do is twist the shaft checking flow with the engine running? I guess thats the most important flow, engine running. |
Tom,
If your carb flows identical for each pair of barrels on a flow bench, then I'd say you have piston ring issue in the "low flowing cylinder". Doesn't take much in blowby percentage loss to affect cfm readings as measured with a carb balancing device like a Unisyn. Twisting the shaft to favour the low cylinder may or may not increase that cylinders flow rate. |
Heres a tip.
When to do the twisting of the throttle shafts? Only after a good run of car with engine properly heat soaked to operating temp. Then shut off and allowed to heat soak for about 10 min so carb bodys get hot. Only then should you twist the throttle shafts. Then allow to cool down for couple hours before re-assessing were your at. This minimizes the metallurgical stresses on the throttle shafts and allows them to maintain there newly twisted position. |
Jonalley:
The UNISYN tool has an adjustment so the restriction can be increased or minimized to allow for getting a similar reading of the tool based upon various cylinder displacements AND idle or desired syncronizing speeds. More precice than the other IMO. I've used both. AND it will allow for fitting on a partially obstructed stack. Best of both world, plus if you decide to sell the Cobra and get an MG it will work on that. LOL......... |
Why does the engine sit under the cowl?
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I think, no I KNOW, that this thread is getting too anal. Please leave the throttle shafts ALONE. It is very unlikely that the throttle shafts are twisted. But if they are, it is the result of over torquing the throttle shaft nuts as previously pointed out.
A difference of one unit between the the two barrels is perfectly acceptable and should not be considered a fault that needs correction. Not in the carb anyway. A greater difference such as 3 or 4 units is a different story. But even then it's more likely that the fault lies in the engine than in the carb. In that case it IS more logical to check out the carb first before engine disassembly just to eliminate that rare chance that the carb shaft is twisted. With the removed from the manifold it will only take a visual examination to see if both butterflies are shut or open the same amount. Try that before you go twisting the throttle shafts on purpose by over torquing the nuts. Z |
I agree with Zrayr. Brass shafts in carbs are soft metal but it would be more heavy handed assembly than the fault of the carb. I was just trying to insure all the carbs had the same attention from my original post. From what many are saying as long as I pay attention to the middle stacks all should be well.BTW I did inspect my butteryflys and they look to close equally so, hopefully , nothing bent.
Rick- I've ordered a UniSync gauge. O.K. Maybe I'm just being too anal about not actually knowing what that last stack is reading compared to the others.Sorry. *13*- The motor just sits back in the engine bay on Factory Five cars. It's not much and four barrel set ups are fine. It's just the line up of the Webers that puts half of the rear carb under the cowl enough that my gauge will not seal on the stack. I've seen pictures of other models and was surprised to see that variation- Probably not more than an inch but enough.Regards, Jon |
The throttle plates are brass.
The shafts are heat treated metal. |
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