I have recently added a Mikuni to my PD, and the idle seems to be erratic. I can get it tuned in on the trailer and all is good. I usually get it set to around 1150, and no problems. When I run the motor, sometimes it will idle perfectly and other times it will die, and other times it will be high? It is usually low though, and I have to give it throttle to keep it running. But then when I turn it up, it will run high. I am going to spend some time working on it tomorrow, and checking everything to try to dial it in with the HSR Manual, but I wanted to see if anybody has had this problem before. I have always heard about how well the Mikuni idles, so I am surprised I am having this problem, Also, if this helps the bottom end is ok, but it could be better for sure.

fuel/air adjustment screw

no prob bud. i own an mc repair shop and those things will give you the fits if not adjusted properly. just when you think you got it dialed in just right, you ride it and it never idles back down prperly. if the setting is the same as the mikuni hsrs i put on harleys, try it at 1 1/2 turns out from lightly seated.

Will Do! Thx again.

In regards to the Throttle response issue, I was thinking I needed to tweak the Accelerator Pump.

HSR Tuning Manual- Accelerator Pump

"The #1 screw is normally adjusted to establish a gap of about 2mm (0.080 inches) between the white plastic lever and pump rod end. If there is a hesitation just off idle, reduce the gap."

Ques- Do you back out the screw or turn the screw in "reduce the gap" I do have a hesitation, so before I screw this up too badly, I thought I would ask! I am assuming this is the answer, (back the screw out) but I want to make sure I am reading this correctly.

"Screw #1 (see Figure T4), on the throttle lever, adjusts the starting point of the pump stroke. To start the pump sooner (smaller throttle opening), back the screw out. To start it later, turn the screw in."

tighten the screw to reduce the gap. but before you do that, get the fuel/air set exact. this can cause the same problems as an accelerator pump. more than likely it is not the a pump. there is a pretty wide variance in that screw to where it will still run right, whereas the fuel air screw is very sensitive

Cool will do, and good to know. Also- this makes sense b/c I just installed a new SS Intake on Thursday.

BUT- If I had to adjust the accelerator pump, what you are implying is that I should start the accelerator pump later, meaning the poor throttle response is from too much fuel, and the motor is bogging down? I just want to make sure I understand what I am doing, and how these things work together. Sorry for the BS questions.

I have been told to get your engine warm before making any adjustments. By screwing in will give the engine more air, which is what you need by the way things sound. Hope this helps.

OK... I worked on the Carb today, and I noticed when we installed the intake we knocked the choke cable loose, and it looked like the choke was slightly engaged.

Before I fixed the choke, I could literally back the pilot air screw almost completely out with out the motor skipping a beat. After I fixed the choke it is the opposite problem. When I back it out about 2 turns the motor begins to hesitate, and at 2 1/2-3 turns the motor begins to die. However, I can tighten the screw to where it is lightly seated, and the motor will still run? It will hesitate, but it does not die. It seems the motor idles best at 3/4 to 1 turn out. However, the idle is still pretty rough and inconsistent. Also, since I fixed the choke, the motor is popping at idle (small back fires) no matter where the pilot screw is set.

I need some help, if anybody has some thoughts on this. Any ideas? When you back out the pilot air screw, what are you doing to the air/fuel mixture? I am not sure if the intake is helping the motor breathe or choking it.

what size engine? what do you have done to the motor as far as mods go (exhaust..cam..etc..)? what jet sizes are you running? what intake do you have? where did you get the carb from?

make sure you adjust it when the motor has been ran and is hot

PD 36, Head work, over sized seats and valves, dual springs, SS Intake Gecker, Gecker Cam, BPS Mikuni, Gecker exhaust. Boat ran great over the weekend. 3 dozen decoys, full hunting gear, 2 guys, grill, 12 gallon of gas, full cooler (50 lbs), CC, 2 batteries, hydro turf through out, gun box, haf box, MB shaggy blind, and five panels of fast grass- I was running 27.5 all day deep- 28-28.5 shallows/hydrilla flats. I am happy with the performance, but the engine is idling like shiat! Will idle at 1500- 800, and sometimes will just die. Thought I could work it out with the HSR Tuning manual, but no luck. I did adjust the accelerator pump to a 2MM gap. I had about an 1/8 of an inch gap!

I almost forgot- I have not changed the jets BPS sent- I think 165 main and 25 pilot. I hope this helps. Thx in advance!

anytime you do what you did to a motor, its gonna make it hard to idle, mine does it some days and some days not

First thing you need to try is changing the "LOW SPEED JET" to a 22.5. Do this....re-tune the carb. and report back.

Craig- I just ordered both the 22.5 and the 27.5 to be safe. Why is the main jet like 3 bucks and the pilot jet 14 bucks? Thx- I will post after I get the 22.5 installed. I would have gone to a 27.5, so I am glad I asked.

have you changed the notch on the needle?

I don't have a solution, but I certainly admire da problem.

Thx- Like I said, based on the 1/2 way point from where the motor stumbled with the screw dialed in and out being less than 1, I would have gone larger! I will go with the 22.5 and try the test again, and see what happens! I will let you know what happens.

You might want to change the spark plugs just to make sure you're starting with a clean slate in case you fouled them.

And make sure you don't have any air leaks (common cause of irratic idling).

Get you a spray can of brake cleaner....with the engine running spray it around your carb. adaptor and all other connections. If a leak is present the engine will speed up!

I stopped by a local Harley Dealer today, and I was able to pick up a 22.5 pilot. I just finished installing the pilot jet and getting the carb reset. I eventually got the boat idling smoothly at around 1100, and got the popping out of the right cylinder to stop. However, I can still lightly seat the air pilot screw and the motor does not stall. But if I turn the screw out it begins to die after 2 - 2 1/2 turns. I have it set right now at 1 1/4 turns out, which seems to be the best idle setting. I have not run it, so I do not know if the idling problems will continue on the water.

I think there were/are several issues. Number 1, the intake "holes" do not line up to the adapter cleanly. Part of the "holes" are actually not even exposed, they are blocked by the adapter. Not much, but enough. The other issue is with the choke. I played with this a lot, and this seemed to really help with the popping, and smoothness of the idle. I am not sure if it is not seated or what, but right now if i move the elbow of the choke up the motor begins to die. I tried to get it adjusted properly, but frankly I am not sure what to do. I just played with it, until I got it to where the motor was idling smoothly. I know though as soon as I run it and hit something, the choke cable is going to move, and I will have problems again. I think the combination of these two issues are the root of the problem.

How do you adjust the choke? Do I need to disassemble everything, or is there an easy way to check to see if the choke is seated? Thx. in advance!

There is a rubber gasket in the choke mechanism that may be forked up. Unscrew the choke assembly and check it out. That would cause the engine to keep running with the idle mixture screw turned in all the way seated. Also change the spark plugs like Wilson said. If all else fails bring it to GECKER.

Yeah- spark plugs are brand new, they have about 6 hours on them. Problem is I did not buy the Mikuni from Gecker, I bought it from a BPS dealer who I spoke to today. I am dealing with Gecker on the intake that does not line up!

Most of these guys on here have forgotten more about mud motors than I will ever know. However, I had a similar problem that was being caused by a combination of the accelerator pump actuator cam, actuator cam spring, actuator rod, and throttle cable pulley being jammed up with paint.

Because they were jammed up with paint, they were never consistently returning to the "relaxed" idle position. Because we thought we had an idle problem, we kept fuuking with the idle mix and actuator cam settings and created a condition that caused a backfire and a blown intake manifold gasket.

Once everything was repaired and freed up, the carb appears to respond consistently to adjustments and the motor has now cooperated for a grand total of two of it's total twelve hours of life to date. I cringe everytime I turn the MFing key, but I am optimistic.

A couple of months ago I was offered some sage advice from DeadAnimal: "modded motor = junk. u are always gonna be wrenching on it instead of hunting.".... I should have listened!

I agree with Craig on this one, I personally have messed with a couple of these motors and stock briggs motors have many issues that create major problems sometimes. I have a stage one motor with a cheetah cam with 652 hrs on it and still running good.

This right here is what helps you to better understand how they work, just hang in there and you'll figure it out!

Put my engine back together. Gecker sent me a modified stock yesterday, I had at my door today. I also ordered a stock, so i now have three! I put the standard stock on to get a base line. First of all, when I put the the stock intake up to the carb and adapter, it didn't line up...hmm... flipped the adapter around(still attached to the carb, carb is now upside down) and what do you know...it lines up perfectly to the intake! The adapter was installed to the carb upside down(not by me)!! I then lined Gecker's intake up, and it fit perfectly!! Note to self, check the stupid stuff first! I put everything back together and cranked it up- same problem. I started working on the choke again and mounted the choke to the new location I had set up. As soon as I did this, all of the problems disappeared! The choke was mounted too close to the carb, and the curve in the line was keeping the choke from seating properly. Unbelievable.

All of this work, a new pilot jet, two new intakes (really three including the stainless), and it all came down to tweaking the choke!! Now that I know it was not the intake, now i have to take it apart again, and put the modified intake back on. The bright side is I learned a lot about this motor over the last few days! I still need to dial it in, and run it, but it is close.

Thx for the advice along the way, Erratic Idling- make sure your choke is properly seated!!

Good info!