I have a older (86') Volvo AQ225 engine that I've been doing a bit of tinkering with to get running well. I was wondering if anybody knew had ever bumped up the heat range of their spark plugs to get away from carbon fouling? I'm currently running AC Delco MR43T. Debating going to MR44T. The info I've found says AC Delco runs from 0-10 (coolest-hottest)

I rebuilt the carb last winter and over this whole summer I've been adjusting the fuel/air mixture ratio trying to get it right. After a couple times out the spark plugs keep becoming carbon fouled and I'll notice it b/c the engine won't run as smooth and will want to stall when idling around the docks. So over the summer I've kept turning the fuel/air mixture down until now I'm at the point they are both all the way in. I was careful not to apply much force when turning down b/c I know not to damage the needle. With each turn down I have noticed an improved length of time between having to clean the spark plugs so I'm moving in the right direction. Now I haven't run it much yet with the mix all the way in but if I start to get carbon fouling again..what would you think about bumping the spark plug heat range up one notch?

Thanks guys,

Best I can tell the AQ225 is the 305 chevy? That have the Holley on it, or the Q-jet? Sounds to me like your fuel issues go beyond the idle jets and spark plug heat range...

If your floats are a tad high, you could be dribbling gas into the venturi's, and that'll foul plugs, and turning in the idle screws all the way is just a bandaid, and it won't idle good...

Our old Sun Runner had the 350 Volvo (AQ271 I think) that came with the Holley. I don't know how anyone afforded to put gas in that boat the way that carb was set up. Main jets were 72's, and I went down to 68's, and it was STILL rich at our altitude. Since it had the secondary metering block instead of jets, I changed the power valve to one that came on at 2" less vacuum. Made a world of difference, plugs stayed clean, transom stayed clean, ran great...

Our old Marathon had a 350 OMC with a Q-jet, and even after I rebuilt it it wouldn't idle. It's problem turned out to be crud in the idle circuit wasn't letting air thru, the only way to make it idle was to open the throttle enough that gas dibbled down the venturis. That didn't make for clean plugs or good idling either!

It is the Q-jet on a 305 chevy block.

I would try to get the mixtures in line before trying to cobble it up by putting hotter plugs in it.


Q-junks are such a pain to adjust (prob. float level as said earlier), I'm really glad I run a Holley (much easier to work on/with).