v4 crossflow overheat protection logic?

[pcrussell50]

for the short version, see the italicized paragraph below near the bottom. for the long version, read on

even though i have three v4 crossflows, and the factory service manual for each one, i do not have the operators manuals for any of them, so...

... anyway regarding the one i'm talking about now, it is a 1977 140hp powerhead on a mid-80's mid and lower, to get the desirable tilt/trim system that the newer models have.

when i bought it, it had sat for at least a year and a half. like a fool, i fired it up in the driveway on the old gas that was in the tank and it ran and idled great and didn't overheat, [verified by my infra-red temperature gun AND the overheat warning system that tested good as well. it also revved nicely when i goosed her a few times, but i didn't keep doing it because i didn't want to have a runaway on the muffs in the driveway. before i took it out to the lake, i mixed in a bunch of fresh gas with the old gas, and as expected, she fired right up and idled nicely. the problem came when i tried to pour on the coals... i could get no more than 1800rpm max. any more throttle angle and she'd just die out. no overheat alarm, [tested it again]. i knew it would be stupid to try and run her on old gas, so my first suspicion is the obvious one... carbs need rebuilding... so i'm doing just that. they are off and clean, waiting for the rebuild kits to come in the mail.

BUT...

i have heard that if you DO overheat, there is a rudimentary logic system on these v4 crossflows that limits your rpm to 2000 or so, until the overheat clears up. what i don't know, because i don't have the OWNERS manual, only the SERVICE manual, is, what system controls this? is it the power pack? does the power pack have some kind of switchable, rpm-limiting capacity such that when the overheat circuit is on, it trips something inside the power pack and puts it in a rev limiting mode?

the reason i ask is that in case rebuilding the carbs doesn't work, i want to know where to look next.

-peter

 

[emdsapmgr]

Re: v4 crossflow overheat protection logic?

That rpm-limiting logic is called SLOW. It came out in the very late 80's on those ignition systems that had the 35 amp vented alternators. I believe the 89 ignition systems were the first. Only on the V6 engines and only on those ignition systems that had the new single power pack. The twin power packs that were made for so long do not have the ability for SLOW. On your engine, the temp switches in the head only talk to the horn in the control box. When the one/both heads reach 212 degrees, the horn is signaled to turn on. The horn will continue on until the powerhead cools down to 175 degrees, when the horn silences. You will have to look elsewhere for your rpm problem. See what you find when you pull the carbs apart and remove/visually check the jets. It does not take much dried fuel in the carb passages to restrict fuel flow and rpm's.

 

[pcrussell50]

Re: v4 crossflow overheat protection logic?

thanks for that. glad i don't have to worry about chasing down a nuisance fault in some old rev limiting scheme. seems like i can always count on you for my v4 crossflow questions.

i'm sure the carb rebuild will fix it as it's behaving exactly like a motor that won't or cant transition to the main jets. btw, i read your profile and it says you are into hydrostreams, too. well the motor we are talking about in my question is on a '77 viper:
recored [of course]
raised transom
sharpened strakes
filled and blueprinted pad
wiseco pistons to fix high ring problem
custom 5-bunk trailer with a 2x6 bunk for the pad to sit on and tow 2x4's on each side.

i also have a '93 115 with VRO just sitting on a stand that i keep around as a spare, and a '75 135 that has been HUGELY reliable, on a 16' sidewinder/tahiti/baja clone.

-peter