Strange AFR problem while cruising...

[jantheeven]

Heroes,

Yesterday, I FINALLY reassembled my good ol' standard card (.9mm a/c, 1.2mm 2nd stage, std main jet) and took the gp A carb out. Rubbish. But.........
I was cruising on the freeway, doing 80 mph, AFR was fine, in the high 14s. But... it slowly decreased to as low as 10 over the course of about 5 minutes. Then the engine started to protest and splutter. I decided to put the foot down and reach maximum boost (21 psi) for a few seconds. After that, normal cruising AFR was fine again, back into the high 14s! But of course... it started to decrease again like before... which took a few minutes again. Put the foot down again and let the turbo scream... problem solved again. This happened over and over. What the hell could be wrong here??

[Sparkie]

is it cold over there?

try putting cardboard over the intercooler, a warm air feed and taking it for a drive.... ;)

[Chris Hebden]

Maybe, acc pump arm stuck?

[Sparkie]

Maybe, acc pump arm stuck?

how is that gonna cause this problem? the accelerator pump only comes into play when accelerating, not cruising?

the cold dense air entering into the turbo inlet is further cooled by the intercooler. the fuel that is sprayed into the inlet air by the carb, doesnt get hot enough to vapourise and hence enters into the engine as droplets, which then wet the plugs. by blocking the intercooler and letting warm air into the turbo, you are then allowing the warm air to vapourise the fuel correctly.

[TrixNFlix]

how is that gonna cause this problem? the accelerator pump only comes into play when accelerating, not cruising?

the cold dense air entering into the turbo inlet is further cooled by the intercooler. the fuel that is sprayed into the inlet air by the carb, doesnt get hot enough to vapourise and hence enters into the engine as droplets, which then wet the plugs. by blocking the intercooler and letting warm air into the turbo, you are then allowing the warm air to vapourise the fuel correctly.


A nice theory. :agree:

[Chris Hebden]

how is that gonna cause this problem? the accelerator pump only comes into play when accelerating, not cruising?

the cold dense air entering into the turbo inlet is further cooled by the intercooler. the fuel that is sprayed into the inlet air by the carb, doesnt get hot enough to vapourise and hence enters into the engine as droplets, which then wet the plugs. by blocking the intercooler and letting warm air into the turbo, you are then allowing the warm air to vapourise the fuel correctly.

I had posted that BEFORE i saw your comment Sparkie which makes much more sense than mine. It was just something i had experienced, accelerated to over take a lory, pulled back in to cruise and couldnt rev out. Acc pump are was stuck in.

[jantheeven]

Allright... thanks so far. No it couldn't be the accelerator pump... I've just reassembled the carb and overhauled it, making sure everything worked fine.
Nice theory indeed about the air temp, but it's not that cold here... around 5 deg C. I do have a bigger FMIC, with which I haven't driven before with these temperatures.
So Sparkie... you think that when I put the foot down and let the turbo reach max boost, hence heating it up pretty fast, also makes the temp in the carb rise, solving the problem temporarily?

[Andrew Cooke]

Allright... thanks so far. No it couldn't be the accelerator pump... I've just reassembled the carb and overhauled it, making sure everything worked fine.
Nice theory indeed about the air temp, but it's not that cold here... around 5 deg C. I do have a bigger FMIC, with which I haven't driven before with these temperatures.
So Sparkie... you think that when I put the foot down and let the turbo reach max boost, hence heating it up pretty fast, also makes the temp in the carb rise, solving the problem temporarily?

5C is plenty cold enough if the weather is damp.

[Sparkie]

yeah, various gtt's of mine have done this over the years, usually on a motorway. most annoying- especially when you come wailing past people who overtook you a few miles back and then suddenly slow down. and you know they are thinking you're a typical boyracer...

[Sparkie]

I had posted that BEFORE i saw your comment Sparkie which makes much more sense than mine. It was just something i had experienced, accelerated to over take a lory, pulled back in to cruise and couldnt rev out. Acc pump are was stuck in.

that would most likely have caused a dig lean spot, and then would have immediately gone back to normal. - don't forget the accelerator pump acts like a filler between the different circuits on the carb and like a fudge factor buffer when you snap the throttle open quick .:agree:

the clue here was that he was cruising.

just call me sherlock holmes.

[jantheeven]

Allright... thanks so far. No it couldn't be the accelerator pump... I've just reassembled the carb and overhauled it, making sure everything worked fine.
Nice theory indeed about the air temp, but it's not that cold here... around 5 deg C. I do have a bigger FMIC, with which I haven't driven before with these temperatures.
So Sparkie... you think that when I put the foot down and let the turbo reach max boost, hence heating it up pretty fast, also makes the temp in the carb rise, solving the problem temporarily?


Okay... but again (Sparky ;))... is my assumption I mentioned here correct?

[Scoff]

I'm with the sparkster, almost certainly carb freeze.

juddery cruising eventually leading to no drive at all if it's severe enough. pulling over and letting it idle for 5 minutes or giving it some stick usually resolves the problem, for a while.

[Sparkie]

wouldn't happen if you had injection.... :laugh:

[jantheeven]

Agree... but then again... with injection it wouldn't be a genuine R5GTT now, would it? ;) Those things are supposed to have their "charming little problems" :wasntme:
And in my case, it's easily fixed... and the fix is fun too... putting my foot down and thus heating the system up a bit ;)

[dave r5]

wouldn't happen if you had injection.... :laugh:


:agree::agree::agree::agree: