> I will say, regarding my NO levels that it passed at 25 mph and 1254
> RPM, but failed at 15 mph and 1287 RPM. Why, I don't know. HC and CO
> readings are fine.
> Pushing open the EGR valve by hand at idle makes the engine stumble
> but does not stall it, so that may be part of the problem, but I think the
> high combustion temp is mainly being caused by carbon buildup. Seafoam
> maybe will help?
> My Olds is a 1985, the last RWD Delta 88. The 307 continued to be used
> in Cutlasses for several more years.
> My mistake, the carb is an electric Q-Jet, not a TQ, certainly not EFI
Ray, because you get good numbers at the 25 MPH test, but not at the 15 MPH test, there are a couple of things that come to mind.
Spark advance occurs more at the low throttle range: perhaps it's over-advanced (or over-advancing: wrong vacuum diaphragm). Or the TCS (transmission controlled spark advance) system is inactive. I can't recall if your Delta 88 had TCS or not, but when it works it's supposed to kill the vacuum advance until the 200-R4 is in high gear.
More likely, the EGR flow is impeded. This is the dreaded (and common) intake manifold EGR passage plug-up. You can't really just pull the carb and run reamers or wires down the ports, it doesn't work. You have to pull the intake and clean the ports in the heads. And it's heavy (unless you have the cast-aluminum intake, but I don't think the 307 got one (?)).
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Years ago, one of my friend Doug's THREE 1986 Buick Estate Wagons was exhibiting very low power and a HUGE stumble off-idle. It had good power up to about 30 MPH, then tapered off and had major problems getting up hills.
The problem was a melted-down & plugged catalytic converter. The high exhaust pressure from the plugged cat was overwhelming the EGR valve and as soon as you tipped in to the EGR port on the carb's throttle body, and a vacuum signal was sent to the EGR valve to open, WHAM . . . a huge squirt of exhaust would go in the intake, and the car would fall on its face. One learned not to dwell in that part of the throttle's range!
The cat had been installed a year before by a muffler shop, to get the Wagon to pass Oregon emissions testing (which it did). It was a dinky, too-small cat. We successfully got the muffler shop to warranty the cat: problem solved.
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On the SeaFoam: lots of people swear by it, but I've never gotten SeaFoam to make much difference. Mopar included a can of some intake/combustion aerosol cleaner with a kit they provide to fix a common problem where some Mopar V8s with plastic intake manifolds experience a backfire and crack the intake -- on the
bottom, so you get phantom oil consumption because the intake sucks the oil from the lifter galley.
Because all that lube oil burning carbons up the piston crowns, valves, and combustion chamber, Mopar's fix kit includes the cleaner. It smells exactly like aerosol paint stripper!
I've heard good things (in the Onan forum of
SmokStak.com), that Onan's aerosol Combustion Chamber & Carburetor Cleaner ("C4") works well for carboned-up Onan gensets. I bought four cans of the stuff last year but haven't had a chance to try it out on anything.
Link to Cummins/Onan blurb:
Onan wrote:4/C Combustion Chamber and Carburetor Cleaner
#326-5278 Our Price: $6.00
Removes power-robbing buildup of carbon, gum, and varnish. Regular use can extend intervals between combustion chamber carbon maintenance cleanings. 12-oz aerosol can provides carbon removal for one genset.
I paid more than $6 per can, because I was buying Onan parts from some other place and just happened to see that they sold the 4/C stuff too.