Surging, low speed miss

[Bob Myers]

Fresh plugs, 3 cans of SeaFoam over last 6 tanks of fuel, checked carb synch with my mercury gage setup and it appears close. Idle is smooth and appears to be same temp 3inches from the head on the pipe. Time to get serious I gues.

From above idle to around 2800 it surges and I can actually hear a cylinder laying down, not real sure if it is same one very time, sounds like it may be on right side though.

Steady throttle around 2600 it will surge really bad, running alongside of a car that appeared to have cruise set it would pick up and start to pass, then die down and run steadily beside, then pick up again. Drop a gear to get above 3k and it disappears, runs steady and smooth.

What is next order of business, diaphragms?

[Power Trippin]

I'm not sure what year bike your referring to, but my 83 does not like to run at a steady speed below 3k rpms. What I mean is, it really runs good above 3k. Below that it seems to have a vibration. Not sure if its in the engine or wheels, or if its a normal characteristic of the bike. Maybe someone will chime in about this. It starts and runs great so I have never thought twice about it. I guess what I'm trying to say is, make sure that it is not normal, before loosing your head over it. Over the years I've had my bike, I have learned that it is finicky, but I love it anyways.

[Peder_y2k]

You made no mention of pilot screw adjustment. This pilot screw affects the slow speed circuit that contributes into mid range. If they are not set accordingly, poor acceleration and driveability are affected.

Diaphragms are easily checked by removing the covers and closely examining them. Tiny holes are inconsequential, but rips and tears require replacement.

Also check the slides for ease of movement. If they are sticky, then that affects driveability during acceleration. Easily cleaned with carb spray.

-Pete, in Tacoma WA USA

[Bob Myers]

I'm not sure what year bike your referring to, but my 83 does not like to run at a steady speed below 3k rpms. What I mean is, it really runs good above 3k. Below that it seems to have a vibration. Not sure if its in the engine or wheels, or if its a normal characteristic of the bike. Maybe someone will chime in about this. It starts and runs great so I have never thought twice about it. I guess what I'm trying to say is, make sure that it is not normal, before loosing your head over it. Over the years I've had my bike, I have learned that it is finicky, but I love it anyways.

 

I'm sorry, I thought my sig line reflected that mine is an 86 Venture Royale'. I will correct that .

Edited May 20, 2014 by Bob Myers

[Bob Myers]

You made no mention of pilot screw adjustment. This pilot screw affects the slow speed circuit that contributes into mid range. If they are not set accordingly, poor acceleration and driveability are affected.

Diaphragms are easily checked by removing the covers and closely examining them. Tiny holes are inconsequential, but rips and tears require replacement.

Also check the slides for ease of movement. If they are sticky, then that affects driveability during acceleration. Easily cleaned with carb spray.

-Pete, in Tacoma WA USA

 

On the off chance that it is the diaphragm, I ordered a set from Sirius Consolidated, new price has risen in four years since someone mentioned they were the place to buy, now-today price is $76.95 per set.

It was running smoothly, last year and the cap over the screw is still intact. If I take them off for any thing I will open that screw up to 2½ turn from light seat.

[Peder_y2k]

Re: the pilot screws, 2-1/2 turns out is a std place to start tuning. My method uses listening to the engine. Thoroughly warm up the engine, it is ready when the cooling fan comes on. If there are no carb issues like partially blocked emulsifier tubes or leaking main jet o-ring, then the engine will respond properly to tuning the pilot screws. Begin by setting idle speed to 1000rpm, then turning 'in' until the engine begins to slow, then turn 'out' till the engine speed picks up and continue turning till the engine slows again, then repeat and set about the middle between the two slowdowns, and then move on to the next carb. Reset the idle speed to keep it around 1000rpm. Listen to the exhaust and sense the engine smoothness as you repeat. It may take half dozen sets to get very close to perfect.

I've adjusted my carbs to the point where there is zero driveability problems ( I can take off in 1'st gear from idle with 2-up and acceleartion is linear), smooth stable running at any rpm, excellent fuel economy (if 47mpg 2-up mountain riding counts), and, the exhaust temperature at each tailpipe is so low that I can hold the back of my hand 3" from the pipe without any burning sensation, warm, but not hot enough to pull away.

All this takes a little while, but the engine NEVER overheats at idle (or climbing in the mountains), and the outcome of my efforts is fabulous performance. As long as the carbs are full of fuel, it starts cold first time, every time, at 1 second cranking

BTW, my scoot has some 52,000 miles on the clock.

-Pete, in Tacoma WA USA

[Venturous Randy]

Sounds to me like you have one or more pilot jets that are clogged.

I think peder is referencing the air/fuel mixture screws, which are not the pilot jets.

Also, small holes in the diaphragms can have an impact on the bike.

RandyA

[dozer520]

i'm jealous! I can't get near that mileage... i'm down in the low thirties and I get the big bang on decal after a I roll on it hard... i'm guessing a clogged pilot as well. but I need some help on what to look for.

 

where are the air/fuel screws located?? thanks

[Bob Myers]

Update;

Ordered 4 new diaphragms from Sirius Ind. on Wednesday, they came on Friday. Now that is service!!

Pulled the old ones out, one at a time, left 2 were in really good shape, soft and pliable, no holes that I could see with a strong light. While I was in there I used the keeper washer to space the needle up and put a different washer on top. Raised the needles app .028". That is about half the distance between slots on my Vulcan needle, and one slot to the next makes a big difference in the fuel mileage on it.

Right side the diaphragms were some what less pliable, one had several tiny holes, about the size of a 20ga needle, the other on had 2 even smaller holes. We were expecting company so i didn't get to ride, but it does sound smoother coming off idle and revving to about 3k.

We'll see today.

[Bob Myers]

Rode the bike to work today(47º,,brr) surge is still there, performance is off considerably. Throttle is not as "snappy" as it was prior to diaphragms and raising needle .028"

Back to the thinking stool I guess.

[Peder_y2k]

Check the engine vacuum pressure sender. It should hold a steady vacuum. The electrical signal from this sender goes to the TCI brain for controlling ignition advance. If the sender is leaking or defective somehow, driveability will be affected. At the #2 cyclinder boot, there is a vacuum port where the hose to the sender is connected, the same port used for carb synching.

Speaking of carb synching, have the 4 carbs actually been adjusted to each other? BIG difference in engine response if not synched in several years.

-Pete, in Tacoma WA USA

[V7Goose]

I have no experience at all with the 1st gens, but a surge is typically a sign of being too rich. Choke partly hung up can cause this.

Goose

[Bob Myers]

Check the engine vacuum pressure sender. It should hold a steady vacuum. The electrical signal from this sender goes to the TCI brain for controlling ignition advance. If the sender is leaking or defective somehow, driveability will be affected. At the #2 cyclinder boot, there is a vacuum port where the hose to the sender is connected, the same port used for carb synching.

Speaking of carb synching, have the 4 carbs actually been adjusted to each other? BIG difference in engine response if not synched in several years.

-Pete, in Tacoma WA USA

I'll put my handheld vacuum pump on this critter tonight and see if it holds steady.

The surge/missfire,whatever is still constant, and still in the same rpm range, app 2300-2500. It is almost like a cyl quits at 23 and rejoins the pulling team at 25. Of course this is where I usually run the engine when I'm in traffic or cruising a slow country road. Of course.

[Peder_y2k]

2300-2500RPM is the transition point from the carburetor low speed circuit to the high speed circuit. The idle mixture screws (pilot screw) functions here to aid the transition, plus control idle mix. If the screws have never been adjusted, then it is time to remove the anti-tamper plugs and adjust them, usually a 1/3-3/4 turn out enriching the mixture. This due to the engine being set for new, and now being well broken-in needs re-setting. Pilot screw adjustment requires the carbs being well synched to each other.

Consider this after the vacuum sensor function is confirmed.

-Pete, in Tacoma WA USA

Edited October 8, 2013 by Peder_y2k

[Bob Myers]

If that is the "point" then likely it is that or a plugged jet. It is like it hits 2300 and a cylinder drops, when it gets to 2500 it comes back in and pulls smooth all the way to the limit of my nerve

[Peder_y2k]

It is easy to determine which cylinder/carburetor is the culprit. Remove the air filter box entirely, then start the engine and warm it up at idle outside in the fresh air. When warm, observe the carbs as you twist the throttle. Try to hold the throttle at the transition point where it has the problem. The malfunctioning carb will either spit fuel at you, and/or the slide will not move as the others do. Either way, it will misbehave in front of you somehow.

It goes without saying....NO OPEN FLAME WITHIN 50 FEET... and don't put your face directly above the carbs, watch from an angle expecting something to fly out at you.

 

-Pete, in Tacoma WA USA

[Bob Myers]

Did that when I put the new diaphragms in, all 4 looked to be responding equally as well as the others.

 

Was talking to a good friend of mine today, the same guy who rode the Iron Butt ride with me. He reminded me this bike was acting almost the same back then, and like this time it appeared sort of suddenly. After thinking on it for a bit i remembered what we did to cure it, then.

Klotz Octane booster. 16 ounces in 5 gallons of gas, ride for 30 miles, let it sit for 30 minutes.

So, i stopped at bike dealership on the way home, bought a couple cans(half off, they are dumping their stock) and dumped half of one in. Rode to gas station, filled up and headed home. i could actually feel the danged thing smoothing out some. Not saying it cured or helped appreciably but,,, take off from stop light and it was pretty smooth all the way up to road speed. Tomorrow will tell me if I was dreaming/wishful thinking or if it really helped already.

Hope it was junk in the jets, I'm not feeling warm and fuzzy about pulling these off.

[Bob Myers]

Well it never got any better, in fact it felt like it was getting worse and since I had bought the wing I just let it sit. Got it out Sunday to ride to work today, it ran just as badly as it did in Oct, the rest din't do it any good. When i got home from work I pulled the diaphrams back out and looked everything over. Made sure the needles were all the same length, etc. I found nothing obvious, so I put it back together. Put the merc vacuum tubes on it and one carb was doing about 90% of the work at idle, it had near to no vacuum while one other was at the top of the scale. So, I did the touch a screw dance for about 10 minutes, tweek, idle, rev, idle, tweek, repeat. I ended up closer to dead perfect than I think I ever saw it. Put it all back together and rode it down the street. Oh crap!!! It about climbed out from under me when I wacked it open in first. Pulled hard all the way to the end of my nerve in 4th(45mph zone too). Still has the 2200 or so surge but at least all four holes are pulling now, and in the same time frame.

Ride to work tomorrow will tell the story, 30 miles interstate, 8 miles of traffic lights, stop/go.

[van avery]

You might try a seafoam soak on the carbs.

Drain the gas from each bowl then with a spray can of Seafoam put the little red tube into the drain line of each ( use a rag to keep down the back spray). Use about 1/4 can in each carb and close the drain valve and let it sit a couple days. Might save pulling the carbs and cleaning.

Be ready to change the plugs again. I also run a couple of tanks of preminum gas after the soak and it helped.

Also, check the ends of the plug wires. Screw the spark plug cap off and check the connection. I had a corrosion problem in one.

Good luck.

[Ben Rollins]

How did it ride today Bob?

[Bob Myers]

Wife rode to work with me so I had to drive the Magnum, I'm riding tomorrow even if it rains

Never seen sea foam in spray can yet. I'll have to look for that

[Bob Myers]

How did it ride today Bob?

When i finished Monday night it was idling around 1100 and sounded pretty smooth, short ride revealed it still has surge around 2200.

Today I rode it to work, nasty skies, may rain(I don't care), bike ran like it used to, strong above 3k, will go from 55 to 90 without downshift-in very short order. Filled up with premium fuel, dumped a whole can of seafoam in, rode the 25 miles to work, last 8 or so in heavy traffic, stoplights,etc. I noticed that the idle was creeping up a little, it is now almost 1400, and it has smoothed out even more. I tried to run it at much as possible down low around 1800-2200 so it would be running on the idle circuit mostly, and it seems to have cleared some of it up. Need to run this tank of fuel out and dump another can of seafoam in. Getting it hot(with heavy dose of seafoam), letting it idle, then sit and cool down seems to work quite well for some one too lazy(me) to pull the carbs off

Will back the idle down to 1k and hope it creeps back up some again.

[Venturous Randy]

After a heavy Seafoam regiment, you will probably need to replace the spark plugs.

RandyA

[Bob Myers]

After a heavy Seafoam regiment, you will probably need to replace the spark plugs.

RandyA

 

I have read this many times and yet to understand what it does to them. I have every spark plug in a drawer(yea, some people keep some of the most worthless stuff!) that was ever in this bike, and 2 others. When it comes time to change them I will have an old set to compare to. Knowing what I should look for would be good too.

[Peder_y2k]

SEAFOAM use causes deposits to form on the spark plug because it is cooler than the combustion chamber surfaces causing condensation of the crap heavy gasses. These deposits will short circuit the plugs. I clean the plugs with a spark plug cleaner after any concentrated fuel system cleaner usage.

-Pete, in Tacoma WA USA