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Carb to Manifold Spacer, are these worth using?

7K views 12 replies 8 participants last post by  TR7RVMan 
#1 · (Edited)
#3 ·
Heat Transfer to Carbs.

the best method used is the factory one. The proper sized "O" ring and careful mounting of the carbs on the studs to give at least a 15 thou air gap between the carb flange and manifold. Not only does this method minimise heat transfer but also the carbs are in effect rubber mounted which cuts down on vibration travelling to the carbs. On older models not fitted with provison for the 'O" ring these mounting spacers may be of use.
 
#7 ·
After reading the wikipedia link posted by TT my opinion is that the main properties of "Tufnol" is the oil and solvent resistance. Just scanning through the rest of the article, most of which was way over my head, I did not see anything along the lines of heat resistance or heat dispertion. Maybe an aluminium one would be just as good. I am not recommending it, just conversation.
 
#10 ·
Hi Rod, I've felt dozens of carbs & manifolds over the last year to answer your question for myself.

I can say with strong confidence, these insulators & the fat o-ring air gap of .040-.060" will be the same. So close it doesn't matter.

The thin o-ring gap of about .015" will be a little hotter. Not with motor running, but with motor off after 10 min.

In every case after riding bike 30+ min. on a 100f day, after shut off & sitting 20-30 min they will all be the same. Smoking hot. That is not avoidable.

You are worrying about something that is not really an issue in real life. If the gas is boiling in carb & it doesn't want to start, just give it a good tickle or full enrich on later carbs & full throttle. It will start right up. I've had much experience with this in hot weather. I've started many bikes like this on hot days & coached others on the same hot days to good results.

I originally had the thin o-ring & .015" gap. It was for sure worse. Then Johntioc coached me on the proper use of the thick o-ring & it was better. I current have the thick o-ring & the 1/2 round rubber washer & steel cup washer & steel lock nuts holding my carb on. It works good.

I have the insulator. I had to enlarge the bolt holes as my manifold has stepped studs. I felt it best to use gasket between insulator & manifold & a very thin o-ring in carb groove. No sealants used. I found my studs were a little short. I could not use cup washers & rubber 1/2 rounds. I went to the thick o-ring & have kept it like that since.

My recommendation from all I learned is to go exactly with the original factory sealing 100%. Don't cheat the 1/2 round rubber & cup washers. Don't use little round O-rings in place of the 1/2 rounds. Get the correct parts. Set air gap .040-.060". As I recall I ended up with about .045-.050" gap using a feeler gauge. I stacked some thinner feeler blades to get the thickness I needed to measure. Kind of wiggle carb as you go & I felt it had good tension at the gap I used. The carb will for sure wiggle as it sort of rubber mounted. O-ring will be too big for groove. Hold ring in place with grease as possible. Nip it up slightly & work o-ring in as you snug nuts. As you go the ring will slide into the groove. Takes a little practice, a bent stiff wire will let you reach inner side of manifold to work ring it.

Don
Don
 
#11 ·
Hi,

The linked webpage says it's 26 mm., which means it's for triples, 500 twins, etc., which all use 'em for the stated purpose.

It's undoubtedly possible to find 'em for big twins' 30 mm. or 32 mm. carbs. but then the carb(s). would be rigidly mounted which, given Meriden and Amal had to devise the standard big twins' carb. mounting to cure a problem (fuel frothing or somesuch?), means rigid-mounting would likely cause the problem to recur?

Hth.

Regards,
 
#13 ·
Hi, Lots of the guys in the club run Amal MKI carbs with ridged heat spacer plates. No issue so far as I can tell. I've ridden bikes like that & experienced no running issues. To me they ran the same as the air gap carb. Even on the hottest 100f+ days in city stop lights the insulator & air gap carbs act the same. You need to keep idle rpm up or they will die at stop lights. They boil gas exactly the same after shut off. They restart the same. Mono block carbs do this the same also.

With motor at fast tickover like 1200 carb never gets too hot at idle. Slow idle it will get hot, but takes about 5 stop lights in a row with temp 90-100f+. I keep my idle a bit high all year just to make sure it never dies. It never does. On the hottest days I blip the throttle often. If the idle starts to drop off, I just open twist grip slightly, making idle about 1500-1600. Once you move off the carb cools very quickly. 30 seconds of moving will cool it substantially. Sometimes in town I only get 10-15 seconds between lights & lights are 2-3 min. long.

In real life once you learn how the bike reacts to heat it's not a problem at all.

Regarding where heat comes from motor off. Some comes up the studs. The stud area on carb flange gets hot fast no matter with air gap & rubber under cup washers or with fiber insulator. However... Most if the heat comes up the intake port/intake manifold area. Once you shut motor off the head & cyl. heat wick right into the manifold & into the air inside the manifold which goes right into carb.

The steel manifold on earlier Bonnies don't transfer heat as fast, but no matter... As Beemie finds with the skillet, the steel manifold soon becomes just as hot. So don't worry about it.

There is no cure for this. Again, it doesn't matter. Once you get the bike on the road, you'll see what I mean. Not an issue.
Don
 
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