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Wed May 02, 2018 8:33 am

I noticed my intake manifold under the carb gets a bit frosty. I pulled the manifold to see if the heat risers were clogged. they are clear. is a little bit of frost normal? I noticed the heat risers also are not that hot to the touch. shouldn't they be very hot?

Wed May 02, 2018 8:47 am

What kind of exhaust do you have installed? Many aftermarket exhausts have those ports blocked off. Also as far as I know only the stock exhaust has a low and high pressure pipe so you get flow through the preheat. The others simply create exhaust pulses that fight each other and do not flow through.

Ray

Wed May 02, 2018 9:44 am

the muffler could be an aftermarket I guess. The muffler heat riser ports are not blocked off and I was able to blow into the tubes. Nothing else would cause icing correct?

Wed May 02, 2018 10:57 am

Using the stock air filter with the preheat hose connected will also help prevent carb icing. Ice on the outside isn't really the problem, ice building up inside can significantly reduce power.

Try touching the heat rises pipes next to the intake manifold after the engine has been running a while. One side should burn you if you hold your finger against it and the other side should be warm.

Wed May 02, 2018 11:21 am

brese73 wrote: the muffler could be an aftermarket I guess. The muffler heat riser ports are not blocked off and I was able to blow into the tubes. Nothing else would cause icing correct?
Just to be clear, are you saying the heat riser flange on the header has an open path into the exhaust pipe? Above it sounds like you are describing the heat riser flange/tube on the header being clear, but if there is no opening into the exhaust tubing there is no way for heat to reach the carb.

Not the best pic, but above you can see there is an opening in the center of the heat riser flange but out of the box it is NOT open into the exhaust pipe. You are expected to drill out the opening before attaching it to the heat riser pipe. These come closed in case you DON'T want to make use of the heat risers.

Is your exhaust a stock/factory style?

Or an aftermarket header style?

Wed May 02, 2018 11:27 am

the heat riser tubes on the manifold are clear. the openings from my stock muffler that connects to the heat riser tubes are also clear.

Wed May 02, 2018 5:15 pm

brese73 wrote: the heat riser tubes on the manifold are clear. the openings from my stock muffler that connects to the heat riser tubes are also clear.
If this is the case then your heat riser tubes should be too hot to touch as they are basically small exhaust pipes. But you mention they are not that hot.

Wed May 02, 2018 10:44 pm

Did you check the manifold's heat risers by sight, or did you blow air through them? If it was by sight, it may not be telling you the whole story. They can and do carbon up in the middle. If you don't have an air compressor, take the manifold to a gas station that has a shop. They will probably blow air through them for free. The air test is the ONLY way that you will know for sure that the heat risers aren't plugged. You could use your mouth, but I don't really recommend it :? .

Thu May 03, 2018 10:07 am

I second what Ashman is saying The flanges may be open but there may be no hole in the pipe below the flange- look closely to be sure if you haven't already. Also, the non stock mufflers don't have a low pressure pipe so there is no exhaust flow through the middle of the preheat, as the high pressure pulses keep pushing against each other.

Thu May 03, 2018 11:01 am

I was actually a little surprised when I compared the preheat effect from a fully functional stock (40HP era) system to the less-functional, BUT DRILLED AND OPEN, system on the Baja Quiet Pack style exhaust on my, uh Baja Bug. Yes, it was not as good, but it still did have "some" function.

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