Carb Insulator Blocks

Well, the El Nino weather has been giving some of you grief, but here in Chicago it means XK driving weather began today. This morning I folded up the cover, checked the fluids, put in the battery, listened to the clickety-clack, flipped on the starting carb, one touch on the button and she fired right up. "She's behaving like she's got spring fever, and it isn't even spring." - State Fair. The easy cold starting reminded me of something I posted about a month ago, didn't get any responses, try one more time. I thought I'd ask you chaps for your opinions. On most of your cars there is a spacer between the carb body and the intake manifold, maybe 1/4" (6mm) thick, supposed to keep some of the heat from transferring to the carbs. My '51 120 doesn't have them, never did, and I'm thinking that maybe I should add them, because it is hard to start when it's thoroughly warmed up and the engine has been shut off more than 10 minutes. The carbs get pretty hot. I usually have to switch on the starting carb. I've noticed British Auto USA is advertising them. But I'll have to get longer studs; mine have only a couple of threads showing beyond the nuts. So do any of you also NOT have them, and have you noticed any difficulty starting warm? Do the rest of you that have them think your warm starting is good? - Rob Reilly

Rob - My 140 has the spacers and it starts hot or cold as long as the battery is in good shape. I don't use the starting carb (mine is converted to manual switch) when I think the engine is still warm enough ie. about up to 1/2 hour after full warm up. My question is how does a hot carb cause starting problems. On some cars we attributed hard starting to a "vapor lock". As a matter of fact what is a "vapor lock" in an SU carb?? Is it the fact that as the gas starts to cool in the float chamber a vacuum forms in the top space above the gas causing the level in the jet area to go down (lean out)?? The SU float chamber hangs to the side of the carb so I wonder how much heat is transfered. What about the oil in the dash pot. Is it getting too hot.Is the underhood temperature more of a problem than the insulating spacer? I think I have some extra spacers if you want to try before you buy. I guess I would need to know what size. On the proper length manifold studs; maybe yours just need to be backed out a quarter inch to work. I don't know if you can tell from the part number how long the stud should be. - Regards; Andy Leavitt

I have no "hot starting problem" with my 140 FHC. I do have the insulator blocks on mine. As I recall, my Healey 100 had them, a TR3 had them, too. Is it possible that the car came from a cold climate? I would imagine that if the carb bodies got hot after shut down, it would evaporate the gas from the float chambers...but the electric fuel pump might handle that. Just a few thoughts. - Barry Goldman

i have a 51 without the insulator blocks and it starts very well when warm or hot. my problem is when its cold ---30 or 40. - regards, M. Larsen

Following up on Andy, Barry and MLarsen's responses: The hard starting is worst when the engine has been shut off for between 10 minutes and 45 minutes. The carbs get about as hot as the intake manifold. If it starts at all without the starting carb, it runs really bad for like a minute or two, then smooths out. I'm thinking the fuel must be vaporizing way down deep in the jet? Is that why I need the extra rich mixture of the starting carb added? Then the water circulation cools off the manifold and eventually cools the carbs enough so the fuel vaporizes at the level it should? So then it's too rich and I shut off the starting carb. That's my working theory anyway. Anybody concur, or have any different theories? If the shutoff time is less than five minutes it starts fine without the starting carb. I have the H-6 type SU's with new correct WO2 needles. SAE 30 motor oil in the piston dampers. The float chambers have the overflow vent tubes on top, so I don't think there could be a vacuum above the floats, but I'll certainly check them for blockages. And no I don't have an overheating problem, that was the other Rob in Hawaii. Comments always welcome. - Rob Reilly

The fuel doesn't vaporize down deep in the jet. The bowl and the jet together make up a U-shaped path where the level of fuel in the jet is the same as the level as the fuel in the bowl. If any evaporates in the jet while the engine is off and hot, it will be replaced by the fuel from the bowl which has much more volume than the tiny space in the jet (much of which is taken up by the needle). As soon as the fuel pump is turned on, both levels are restored to the level controlled by the needle vale in the bowl.In operation, the depression (vacuum) created by the venturi effect at the top of the jet sucks fuel up into the throat where it is turned into a mist and the little droplets in the mist vaporize (evaporate) before the gas/air mixture gets to the cylinder. I know this doesn't even come close to suggesting a cause for your problem - it's just a comment on your "working theory" to help in the process of elimination. I had very serious hot starting problems last summer until I rebuilt the distributor. Now I "press and release" the button if the engine has been running any time within about six hours - i.e. I don't have to wait for the engine to start - I just know it will be running by the time I release the starter. The points, capacitor, vacuum advance, and centrifugal advance were all bad. I can't narrow down the cause any more than that. I do know that having your spark retarded makes starting easier. My "29 Chevy has a timing adjustment on the dash and I learned never to try starting it with the hand crank without using it. With the electric starter, it didn't matter much. Either automatic advance mechanism not working properly could contribute to your problem. - Bruce Cunningham, '53 XK120 OTS

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