What is carburetor percolation..., How can I take care of this annoying problem Options

[nsr-jamie]

Hi, over here in Japan where it is much hotter and humid than the rest of most parts of the world. My car is a 1972 914-4 model that has been tuned up a bit with 103mm p/c, high can and big valve heads (48x38) and twin 40mm drla Dellorto carbs....in the summer time the car seems to run badly and when I stop the car for even a short time to run into the 7-11 and try to restart the 914 it sometimes is quite difficult to get it running again. I talked with my mechanic friend at my local air cooled VW shop here and he told me I was experiencing carburetor percolation which seems to be common on these cars or air cooled engines that are running carbs like mine. I did a bit of research and found nothing here on this site but a little bit on google....not really sure what carburetor percolation is but kind of sounds like the carbs are over heating from the engine heat. Are others experiencing this problem too and if so what are you doing to try to fix it. My shop mentioned special gaskets or spacers to go in between the carbs and manifolds to cure this problem...it seems to occur more here on this side of the planet cause of the humid and hotter weather compared to say back home in the US or Europe. I checked on the CB Performance site and could not find spacers.....any body have any input to add? Thanks and cheers

Jamie in Japan

[McMark]

When the engine is running you have air flow and fuel flow to keep things relatively cool. But when you shut off the engine all that heat builds up in the carbs and causes the fuel in the bowls to boil and spill into the carb throat. When you go to restart you have a flooded engine.

Your shop sounds like they know what they're talking about. There are phenolic spacers that will reduce heat flow from the heads into the carbs. This is where a lot of the heat comes from. The stock FI-style intake manifold gaskets are phenolic and you can find the intake-carb phenolic spacers on eBay in the US searching 'phenolic idf'. The DRLA has the same bolt pattern.

[sean_v8_914]

the plastic spacers help. Porsche used a return line off teh fuel pressure regulator, after the carbs. most carb conversions use a pressure regulator before teh carbs. it only allows about 3 psi to the carb. the carbs are a dead end-final destination for teh fuel. Porsche used a regulator after the carbs. any fuel over teh required 3 psi was let out to a fuel return line to keep fuel cool.

the simple solution seems to be teh plastic spacer but this will change your linkage geometry since teh carbs will be up higher and wider apart.

[r_towle]

spacers are the key.

rich

[GeorgeRud]

You can also put a switch in the dash that let's you shut off the fuel pump. Then, simply cut off the fuel flow to the carbs a bit before shutting down. The lowered fuel level in the carbs will also help cut down the percolation.

[mrbubblehead]

hi jamie, i had a similar problem. on super hot days i could actually hear the fuel boiling in my float bowls after shut down. so i use the phenolic spacers between the manifold and carb. as well as the stock phenolic spacer between manifold and the head. i work in the desert, and once my carbs heat soaked, they didnt want to idle real well. once i isolated the carbs my heat soak problems went away.

(IMG:http://www.914world.com/bbs2/uploads_offsite/i722.photobucket.com-12492-1352161647.1.jpg)

[nsr-jamie]

Hello everyone and thank you for the excellent replies. Spacers sound like the way to go and now that my engine is now out of the car I have decided to try to rebuild the carbs myself...I saw the spacers on Ebay (thanks McMark) and I think I might be able to make them myself on the CNC mill at work in my free time. It seems like the material I need is called phenolic and I did a quick search and "BakeLite" was also mentioned. We actually have some bakelite at work that has been sitting for years that I could use and try to attempt to make a set of these spacers. It might be kind of fun to try and make them and see how they turn out.

Is this going to affect my linkage, the thickness of the material is 8mm....I guess I will need new stud bolts for the manifolds.

This could be fun to try to make something like this....just wondering if I can machine or mill bakelite (phenolic), I guess I need to do more searching again on the web. I will let you guys know soon.

['73-914kid]

Jamie, what linkage are you running? I installed the phenolic spacers after installing Chris Foley's cable linkage without any real issue, or need to res-synch. For almost any other linkage though, I think it would throw the geometries out a little.

BTW, I've haven't had any good luck milling bakelight. It chips away rather than milling cleanly. But I'm sure with the right tool could mill it cleanly, but from my experience, it's a crap shoot.

Out of curiosity, would Delrin work as well as phenolic material? Talk about a material easy to come by, and easy to mill....

[john rogers]

Do you have all the sheet metal AND the rubber seals that fit between the tin and body parts? If not then the heat will surely come up, worse when stopped. You can also add some aluminum heat shields over top the exhaust to help with radiated heat deflection. We used to use these on the 4 cylinder race car to help keep heat away from the carbs. With the spacers, be sure to tighten the nuts very evenly and not to over tighten or air leaks will result.

[charliew]

The spacers I have seen have a mesh looking reinforcement running through them. We use a spacer on suby motors that also looks like it has a mesh in it. The stuff I have seen looks just like the stuff electronics hobbiest use to mount electronics on.

[DBCooper]

I don't think you want Bakelite, it cracks really easily so I doubt you'd be able to torque the carbs down and keep it intact. Phenolic has a bit of give. It's just what was used back in the day, but if you're making them I imagine you could use any of a dozen other modern non-metalic materials that would do the same job.

[nsr-jamie]

Seems like lots of guys in Japan are using bakelight with their air cooled cars...today after work I cut 4 pieces of them on the band saw and drilled out the stud bolt holes on the mill today so they are perfectly pitched. I am going to make some kind of jig for them either tomorrow or the day after when work is done and hope to maching a few pairs of them. The material is very soft and I tested with a carbide endmill and it turned out much cleaner than I expected. I'll try to get some pictures this weekend and let others know how they work out. This is kind of fun...

[URY914]

Jamie,

I see a group buy in your future. (IMG:style_emoticons/default/biggrin.gif)

Pictures would be great.

I have this same problem after a few autocross runs. I need to do something.

[Woody]

Jamie,

I see a group buy in your future. (IMG:style_emoticons/default/biggrin.gif)

Pictures would be great.

I have this same problem after a few autocross runs. I need to do something.

(IMG:style_emoticons/default/agree.gif)

[DBCooper]

Seems like lots of guys in Japan are using bakelight with their air cooled cars...today after work I cut 4 pieces of them on the band saw and drilled out the stud bolt holes on the mill today so they are perfectly pitched. I am going to make some kind of jig for them either tomorrow or the day after when work is done and hope to maching a few pairs of them. The material is very soft and I tested with a carbide endmill and it turned out much cleaner than I expected. I'll try to get some pictures this weekend and let others know how they work out. This is kind of fun...

Maybe there's a difference in terminology then, because Bakelite was a brand name and one of the very early plastics, from the first part of the last century. It was black and very hard and brittle, not soft, typically used for molded electrical connectors and components. I wasn't even aware it was still being made. Whatever you call it, if what you have is soft, you can machine and it will insulate then it will work.

[nathansnathan]

Seems like lots of guys in Japan are using bakelight with their air cooled cars...today after work I cut 4 pieces of them on the band saw and drilled out the stud bolt holes on the mill today so they are perfectly pitched. I am going to make some kind of jig for them either tomorrow or the day after when work is done and hope to maching a few pairs of them. The material is very soft and I tested with a carbide endmill and it turned out much cleaner than I expected. I'll try to get some pictures this weekend and let others know how they work out. This is kind of fun...

Maybe there's a difference in terminology then, because Bakelite was a brand name and one of the very early plastics, from the first part of the last century. It was black and very hard and brittle, not soft, typically used for molded electrical connectors and components. I wasn't even aware it was still being made. Whatever you call it, if what you have is soft, you can machine and it will insulate then it will work.

I too had thought bakelite was not made anymore, but bakelite and phenolic are just about the same thing according to wikipedia, bakelite being the trademark for the first completely synthetic plastic, a thermoset made from a reaction between phenol and fermaldehyde. I used to work for a lady that designed retro jewelry, and there is a whole crowd of old ladies that are into the bakelite bracelets and such, they would rub them and smell to check the authenticity - will have to try that on some spacers next time (IMG:style_emoticons/default/smile.gif)

[larryM]

go check the PMO site for info

- in addition to insulating spacers for webers, there was an anti-percolation mod done in the carbs

yes, i know you are not talking weber - but carbs are carbs - you can learn from that

as i recall it was caused by EPA changes in the volatility (reid vapor pressure) of our enviro-approved gas

- special tool as i recollect, and i think i still have the thing

i did all that yrs ago on my sixer

.

[rhodyguy]

are the spacers between the idfs and intakes cut down intake to head spacers? i've never seen that application before. where did you purchase them?

k

[nsr-jamie]

Jamie, what linkage are you running? I installed the phenolic spacers after installing Chris Foley's cable linkage without any real issue, or need to res-synch. For almost any other linkage though, I think it would throw the geometries out a little.

BTW, I've haven't had any good luck milling bakelight. It chips away rather than milling cleanly. But I'm sure with the right tool could mill it cleanly, but from my experience, it's a crap shoot.

Out of curiosity, would Delrin work as well as phenolic material? Talk about a material easy to come by, and easy to mill....

I am running what I believe is the CB performance hex link kit. I bought the whole kit with the 40mm DRLA dells from Dave Shepard at Mid Engine Mania (now GPR) around 15 years ago or so new. I have also been thinking about upgrading to the Tangerine linkage too someday.

I am not sure of the actual name of the material but everybody is calling it bakelite at my work...I did some research and heard it was not even made anymore but I also heard Mitsubishi Heavy Industries makes it....from the pictures it looks just like the stuff sold on Ebay as Phenolic so it may be that. It gives off lots of powder when you machine it and a bad order...I wore a mask for this...I have a feeling this is not a material you want to machine much

[nsr-jamie]

Ok, here are some pictures as promised. I finished one set this week.

Picture one shows the material I am using which may be bakelight...I actually got it for free from our spot welder operator as he no longer needed it. Thats how I got it with the holes in it already.




Another one but close up. Is this really bakelite or is it Phenolic? I believe they are the same material.