I'm finding my wagon, with Mikuni's, is simply not as well-mannered when cold as my sedan with SU's; it doesn't matter what I do with the carbs, increasing the richness any further leaves the car running too rich when fully warmed up.

Perhaps I'm expecting too much; the last time I drove a carbed vehicle with something other than SU's was over 20 years ago, and I recall having to let it (74' Ford Econoline with 302) warm up for several minutes before the thing would drive without bucking and sputtering.

With SU's I've always been able to drive the car immediately after starting, regardless of the temperature, without a lot of fuss. I thought the Mikuni's were supposed to be so wonderfully advanced; the car really does get up and go with the engine fully warmed up, but it would have been nice if the Mikuni's were an improvement over the SU's in every possible way; instead I have to play games with the accelerator pump, and open the throttles just right... PITA

I'm starting to look at other things on the motor I might be able to alter to help the engine run better when cold (aside from putting the Mikuni's on ebay).

The car with Mikuni's has the asymetric five bladed fixed fan; it also has a ceramic coated exhaust manifold. I'm guessing neither of these features are helping cold weather performance. I don't want to get rid of the exhaust manifold, or the aluminum intake.

The first thing I can think of is to put an electric pusher fan with thermostat control in front of the radiator and getting rid of the fixed fan entirely.

The other thing I thought I could try is to put in a higher temperature thermostat.

Beyond that, the only thing I can think of is to try and fabricate some kind of air preheater on the intakes of the carbs, and maybe a radiator shutter of some kind.

It isn't all that cold where I'm at; low 30's to mid 40's

I'm sorry I didn't stick with the SU's, to be honest; information and support about Mikuni's for Volvo applications is simply too thin on the ground. I have no adequate frame of reference for knowing what is normal and what isn't with this carb setup.

Bill

SU carbs with "economy" needles would act like this.
I don't know enough about Mikunis to say much here but I agree pretty much
with the other guys. If they have a rich mixture device for starting, USE IT!!
(sometimes erroneously called a "choke")
But if it idles OK and is OK at sustained high speeds but only bucks during
acceleration it sure sounds like a needle problem, assuming no throttle-shaft
leaks.
One thing I did on my 164 when returning from Panama in 1998 was to refit the
flapper valve and air pipe that gathers air from the area of the downpipe whenever
the temp is below 70°F. That alone helped a LOT in cold-weather driveability,
without carb adjustments, etc.

One other thing that you probably will not do is to use the '67 combined cast
arn manifold, which is a GOOD as long as the secondary butterflies either work
correctly or are removed. This warms low-speed air within seconds of startup
without materially affecting high-speed operation and can really enhance cold
weather driveability. Although this manifold has received lots of bad press,
probably because of problems actuating the secondary butterflies, it actually
flows pretty well on both sides and was used on the highest-output B18s.
At high-speed full throttle operation the air passes through so fast that the
charge fairly well cools the skin of the intake passages and from that point the
charge passes through so fast that it doesn't pick up much more heat.
--
George Downs, Bartlesville, Heart of the USA!

Bill;

Just like SUs, Mikunis are essentially also a constant-velocity/variable venturi design aren't they...so it seems to me that they would similarly like (lots of) choking during engine warm-up (don't they have a provision for this?), a richer winter setting, and pre-heated air when its cold...all the things that we know SUs like when its cold...so exactly why did you change? ...was there really that much perf to be gained in the summer? ...sounds to me, as Shannon has already stated, that you may need to rejet for the winter...and maybe an airbox, taking carb-air from the exhaust manifold (from a 140)...what a pain...not to rub it in, but I just use my good-old manual choke, and go 3 flats down on the jets November to March...it costs $0 and I can do it at a red light if necessary, without missing the next green!

Cheers

Howdy spectratone,

You need to rejet your Mikunis. There is little to be gained by changing the thermostat or fan or anything except the jets. You could build a stove box around your header and use it to warm the intake air, but beyond that there is little that can be changed external to the carbs.

I am pretty sure several snowmobile manufacturers used Mikuni carbs, so it is not a hopeless task, only one you will need to learn how to do and then do it at least twice a year; maybe more often if you live in an area with a wide temperature swing.

http://www.jetsrus.com/main_page.htm
--
Mr. Shannon DeWolfe -- (I've taken to using Mr. because my name tends to mislead folks on the WWW. I am a 51 year old fat man ;-) -- KD5QBL

I guess I have been misled by other's reports on their Mikuni's.

Folks here were reporting using them right out of the box with no changes. I've set the standard needles to their richest setting; I also have the chokes connected and working.

In desperation I even contacted the guy that sells Mikuni HSR kits for Triumph sportscars (since you-know-who doesn't answer emails), who lives about three hours south of me, and he stated that the carbs don't need seasonal adjustment (in his area?) and that the chokes and accelerator pump provide enough leeway under cold conditions.

I'd like to see a picture of the system that Volvo used for air heating; I had been thinking of constructing an air cleaner assembly that could pull air from inside the engine compartment or from behind the front grilles.

The only other thing I can think of would be yet another air leak in the induction system; although I have dismantled the entire induction system between the backs of the carbs and the ports on the head, and regasketed/resealed everything, which made a noticeable improvement in performance.

Could there be any issues with using a B18 crankcase ventilation setup on a B20 block?

Bill

Howdy Bill,

Sorry, I don't have a picture of the preheat setup. George's idea of using a combination manifold with the butterflys removed is probably the best idea.

I've set the standard needles to their richest setting...

If you have read the Mikuni Tuning Manual, you know that the pilot jet is going to control the mixture up to about 1/4 of the throttle travel. When you move the clip on the needle, you are making the main circuit much richer for the same throttle opening but have not effected idle or off-idle very much if any.

Set the needles back to whatever gives the best performance and best transition from pilot circuit to main circuit. Get the numbers off of the jets and order + and - sizes of both the pilot and main jet. In fact, you might want to go ++ on the pilot jet since you indicate that it doesn't start and idle well at all.

If the Triumph guy says that once jetted, seasonal changes don't require rejetting, that may well be true. But, the jets that you have may not be right for your setup. I am willing to wager that a change in the pilot jet size will help, maybe even improve the summer performance too.

Just a thought: If you can afford a day off work and a trip to this man's shop, make an appointment with him to help you setup your carbs properly. Someone who does this for a living will have the equipment, experience, and the jets to do it right. If you don't live significantly higher or closer to sea level than his shop is, it should be a done deal at that point.

I vaguely recall your thread concerning air leaks. Do you suspect another leak? Or is it more a case of frustration over the carbs?

I would not fret over the PCV setup. It should be OK.

Let us know how it goes.

--
Mr. Shannon DeWolfe -- (I've taken to using Mr. because my name tends to mislead folks on the WWW. I am a 51 year old fat man ;-) -- KD5QBL

Bill;

Beware of statements by people looking to sell you something...it's best to get a reference from a (hopefully happy) customer who isn't his brother-in-law...and if Brickboarders have given you a good review, I expect you are in the ballpark, but when it comes to carbs, there's always some amount of fine tuning required...so I have some trouble believing the statement "right out of the box with no changes".

Sorry, I can't help with a pic of the warm-air induction system...

I expect you are "pulling air from inside the engine compartment" now, in which case it would be pretty cold...and certainly would be if you were to draw it "from behind the front grilles", where it gets zero preheating. For winter, you could maybe get a hold of one of those 140 systems, or otherwise cob something together out of cookie tins and that metalized bellows tubing, taking (preheated) air from the downpipe.

Regarding the PCV, there's no inherent reason not to use a B18 system on a B18, BUT, they are different and they do require a calibrated orifice for flow limiting...if this is not present, it will act as a vacuum leak and maybe indeed be responsible for lean symptoms...refer to: http://www.sw-em.com/pcv_diagrams.htm

Cheers

Hold on a second, are you saying I need a nipple on the intake manifold with a restricted orifice if I use a B18 style PCV setup on a B20 block?

The intake manifold is definitely not equipped with that part...


Bill



Quote:"Regarding the PCV, there's no inherent reason not to use a B18 system on a B18, BUT, they are different and they do require a calibrated orifice for flow limiting...if this is not present, it will act as a vacuum leak and maybe indeed be responsible for lean symptoms...refer to: http://www.sw-em.com/pcv_diagrams.htm"

If you have a PCV valve, you don't need or want the restricted manifold fitting. That is used instead of a PCV valve.

Quote: "If you have a PCV valve, you don't need or want the restricted manifold fitting. That is used instead of a PCV valve."


Okay, that clarifies things; the B20 engine should be plumbed correctly (as a B18) then.

I just checked the engine vacuum again, which I haven't done since replacing the intake/exhaust manifold gasket (which did make the engine run noticeably better).

It's still reading around 16.5 inches... If I increase engine RPM's it climbs no higher than 20 inches.

I did try replacing the PCV valve, as well as all the hoses. Could a leak in the breather box, or the breather box to block o-ring cause low vacuum, or would the PCV valve prevent that from happening?

The engine only has about 1000 miles on a complete rebuild, could frictional drag be giving me a low vacuum reading? or should I go back to looking for vacuum leaks again.

Is it possible for the aluminum intake manifold to become porous if it is polished? I did smooth the outer surface to an almost mirror finish as part of the polishing process. I'm really grasping at straws since I've gone through the induction system multiple times now.

Bill

Bill

16 inches vacuum at idle is perfectly fine. You'll only see more than 20 on the overrun (throttles closed) after you race the rpm up or decelerating from speed while driving. Spray a water mist (or carb cleaner if you're braver) around the intake/carb mounting area with the motor running. A vacuum leak there will show up as changed idle speed.

It's still reading around 16.5 inches...

Good morning,

Have you adjusted the valves?

sd

I've adjusted the valves, gapped the plugs, tweaked the idle mixture. I also tried two different vacuum gauges, which both gave the same results when connected to the nipple on the intake manifold.

The entire ignition system has been replaced or refurbished, the carbs were new when installed on the rebuilt engine, about a thousand miles ago.

I already replaced one defective thermostat; the new one seems to work normally and the temperature gauge shows what I would expect during warmup.

If 16.5 inches is within the normal range for a freshly rebuilt engine, then I suppose I'll have to focus on the carbs again... it looks like an O2 gauge will have to be purchased in the very near future.

Bill

Bill;

I'm saying that unrestricted, it will act as a vacuum leak which will lean out the mixutre due to all that false air...a quick test would be to disable PCV by disconnecting PCV hose from the intake manifold...plug the hole and vent the crankcase to atmosphere...do starting, running, and idle improve?

Good Hunting!