| Author | Message | ||
| Russell Beam (Rcbeam)
Registered Member Username: Rcbeam Post Number: 38 Registered: 11-2009 Posted From: 74.131.50.239  Rating: N/A |
When I bought my MC8 last year the high idle rev'd way too high. It was on my list of things for DD to fix last winter while they had the coach for months, but evidently they didn't. I really want it to idle at the correct 'high idle' speed. I have no tech so I can't tell you what the rpm is but it's really high. Is this something that I can adjust or should I leave it alone and try to find another place to adjust it? | ||
| JC Alacoque (Jc_alacoque)
Registered Member Username: Jc_alacoque Post Number: 98 Registered: 7-2006 Posted From: 207.34.166.7 Rating: N/A |
Fast idle should be around 1200 to 1500 rpm. You can get a hand held tach at Harbour Fright for not much money. Adjust the plunger off the air cylinder, that pushes on the fuel lever at the governor untill you get 12 to 1400 rpm. JC | ||
| George M. Todd (George_todd)
Registered Member Username: George_todd Post Number: 1138 Registered: 8-2006 Posted From: 99.37.31.51 Rating: |
I don't want to be picky, but: The fast idle adjustment actually works on the end of the fuel rack, not on the governor. The air cylinder up on the governor prevents throttle pedal movement on DD3 equipped buses when the high idle is engaged. A slight mis-adjustment of the high idle cylinder can cause a runaway, as the normal operation of the fast idle takes control of the engine away from the governor. If the engine is equipped with Jake brake, the fast idle fitting is different, containing both the high idle cylinder, and the Jake brake buffer switch. The fast idle cylinder has to push on the end of the rack, while still allowing the buffer switch to feel rack position. G | ||
| Russell Beam (Rcbeam)
Registered Member Username: Rcbeam Post Number: 39 Registered: 11-2009 Posted From: 72.250.169.58 Rating: N/A |
When I had the rack set on the 4905 that I used to have, the mechanic said the buffer screw adjustment affected the high idle. On the MC8 it looks like an air hose connected to where the buffer screw was on the 4905. Everything I've found on the internet so far says high idle should be between 1000-1200, but I have no idea where to adjust it. So I'll ask again where to adjust or should I get a DD mechanic to do it? | ||
| Dallas Farnworth (Dallas)
Registered Member Username: Dallas Post Number: 141 Registered: 7-2004 Posted From: 75.91.207.143  Rating: N/A |
Russell, I haven't heard from you in years! Welcome back! IF you aren't conversant with the governor settings, I would take the bus to a good DD mechanic. We got our bus from near you in Versailles, and they seemed to be pretty good with DD's, except they missed some small stuff that I had to fix later. I'm not certain I still have the name of the place or not, but I'll look and see. Dallas | ||
| Russell Beam (Rcbeam)
Registered Member Username: Rcbeam Post Number: 40 Registered: 11-2009 Posted From: 74.131.50.239 Rating: N/A |
Dallas: Yes, I'm still around and occasionally post questions on here. The only DD shop I know of in this area is Republic Diesel. They are the ones that set the rack on my 4905 when I had it. But when I they had the MC8 for months they didn't fix several things they were supposed to. They seem to be swamped with work this year. Was hoping it was something that I could easily adjust but if not, I'll certainly not go putzing around with the governor and mess my sweet 8V up. Didn't realize that your bus came from my neck of the woods. | ||
| Tony LEE (T_lee)
Registered Member Username: T_lee Post Number: 75 Registered: 11-2007 Posted From: 59.154.42.140 Rating: N/A |
On my MC8 the problem was the throttle-lock actuator was sticky and occasionally not extending fully. Dismantling, cleaning and lubricating fixed the problem but I wouldn't recommend selecting fast idle until the air pressure is fully up - because if the throttle-lock actuator needs a bit higher pressure to extend fully, you can get the "runaway" happening towards the end of air-up. | ||
| George M. Todd (George_todd)
Registered Member Username: George_todd Post Number: 1139 Registered: 8-2006 Posted From: 99.37.31.51 Rating: N/A |
Russ, From what I read, your 4905 had Jake brakes, and not a high idle? It also sounds like your 8 has high idle and no Jake? If so, you can start up and air up, and the fast idle plunger has the same effect on the rack as the Jake buffer switch. As the plunger moves in, the engine speed will increase. So, get your wrenches out, switch on the fast idle, and UNSCREW the plunger assembly until the speed slows down to where you are happy with it. Then re-tighten the lock nut. Please be familiar with the emergency shutdown and how to trip it, and that it trips fully, in case something happens and the engine starts to overspeed. Good luck, G | ||
| Russell Beam (Rcbeam)
Registered Member Username: Rcbeam Post Number: 41 Registered: 11-2009 Posted From: 72.250.169.58 Rating: N/A |
George my 4905 did NOT have jakes and it DID have a properly working high idle. The MC8 also does NOT have jakes. | ||
| Dallas Farnworth (Dallas)
Registered Member Username: Dallas Post Number: 143 Registered: 7-2004 Posted From: 75.91.207.168 Rating: N/A |
Russell, There is a fiber washer on the end of the lock nut on your buffer screw. These are notorious for hardening and crumbling into dust, then the plunger can move in a lot farther than intended, raising the high idle. Buy one of those $30 photo tachs on eBay, replace the fiber washer and have at adjustment. Make sure the engine is at operating temp when you do it, or you'll find the rpms are higher than you want the next time you fire it up cold. I can talk you through this when you get to the point that you need to set the buffer screw, just remember, a minor adjustment on the plunger will make a huge difference in the idle speed. Email me from my profile and I'll send you my phone number. | ||
| Russell Beam (Rcbeam)
Registered Member Username: Rcbeam Post Number: 42 Registered: 11-2009 Posted From: 74.131.50.239 Rating: N/A |
Dallas: I do not have a buffer screw as far as I can see. What I have is some sort of air cylinder with an air line hooked to it where the buffer screw would be. The air line is T'd off of the supply for the high idle valve on top of the gov. So when one gets air the other gets air. Don't know if I can just screw the air cylinder out or in a turn or two to change the high idle rpm or not. | ||
| Dallas Farnworth (Dallas)
Registered Member Username: Dallas Post Number: 144 Registered: 7-2004 Posted From: 75.91.207.168 Rating: N/A |
Russell, that cylinder is the buffer screw actuator. The buffer rod (or pin) is inside the screw threads that you see. Do you have the DD manual for your 8V71? | ||
| Tony LEE (T_lee)
Registered Member Username: T_lee Post Number: 76 Registered: 11-2007 Posted From: 59.154.42.140 Rating: N/A |
Since the idle is WAY too high, it seems as if the lower cylinder is extending as it should. Before you change anything, make sure the top cylinder is extending fully and locking the throttle in the "pedal up" position. You can check this from the drivers seat. If it is reving too high and you can depress the accelerator, something is wrong. Pull the pedal up by hand and the engine revs should drop to the correct value (provide there is not too much slop in the linkages. Another way to check is to depress the throttle a bit so it is revving at about the correct high idle speed. Flick the switch to high idle and the throttle should be pushed up hard against your foot. | ||
| George M. Todd (George_todd)
Registered Member Username: George_todd Post Number: 1140 Registered: 8-2006 Posted From: 99.62.11.168 Rating: N/A |
Lets stop and THINK about this for a couple of minutes. 1. High idle on the two 8s in question, and my 6 is air operated. COULD the speed cylinder which is teed in parallel with the throttle blocking cylinder extend and increase the engine speed before the throttle blocking cylinder extends fully, AND PREVENTS THE THROTTLE PEDAL FROM INCREASING THE ENGINE SPEED REQUEST TO THE GOVERNOR? Well, I guess so, but then the engine would alreay be revved up, with or without air, if the pedal or mechanical linkage was stuck. 2. If that were the case, the overly high idle would settle down within a minute, due to the engine's high speed building air quickly. 3. Russ isn't saying ANYTHING about high normal idle speed! 4. NONE of this applies if the bus has an air throttle! 5. After reading all of the above, maybe we re-think the runaway comment above when the throttle blocking cylinder "...doesn't extend fully under low air pressure..." (The way I see that situation, the throttle would have to be depressed during the very narrow time frame while in high idle with somewhat low air pressure.) Then the governor still and always has control over how far the rack is OPENED, it just can't CLOSE the rack beyond the high idle position. G | ||
| Russell Beam (Rcbeam)
Registered Member Username: Rcbeam Post Number: 43 Registered: 11-2009 Posted From: 74.131.50.239 Rating: N/A |
My low idle speed seems normal. When I first bought the bus sometimes the high idle would work right but mostly it would just rev way up. A few times it ran at normal high idle for a minute then rev'd way up. The shop was supposed to fix that but evidently they did not. Ever since I got it out of the shop last spring the high idle is always really HIGH, like maybe i have the treadle half way down or something. The throttle blocking cylinder plunger appears to me to be extended fully. One other thing I can add here that might mean something to you guys... when I do a cold start the rpm's rev up for just a second before the gov catches and pull it down. Not really high, but the 4905 that I had before never did that. I don't notice it when doing a warm start. One more thing to add. The lower cylinder at the buffer port is really oil coated and dirty. Everything else is spotless. When I get time I will try what George says and play with the throttle and high idle to see if I can see anything that might help to mention. Thanks guys. | ||
| George M. Todd (George_todd)
Registered Member Username: George_todd Post Number: 1141 Registered: 8-2006 Posted From: 99.103.159.139 Rating: N/A |
This is beginning to sound like either a sticky injector, or a governor that isn't pushing back on the rack as strong as it should? G | ||
| Russell Beam (Rcbeam)
Registered Member Username: Rcbeam Post Number: 45 Registered: 11-2009 Posted From: 74.131.50.239 Rating: N/A |
Went out yesterday to try to start the bus to investigate further after reading some of the posts here... batteries were mysteriously too low to crank. Spent the afternoon charging the 8D's. They are 7 years old. Finally got them charged, specific gravity good, but as soon as you load them up, they fall on their faces. Now I've got to order two new batts. Maybe by the weekend I can try this again and get back with you guys on what I find. The last pair I bought was in 2003 and they were $100 each then. I priced them today and same thing from the same place is $188. | ||
| Tom Caffrey (Pvcces)
Registered Member Username: Pvcces Post Number: 1342 Registered: 5-2001 Posted From: 65.74.72.213 Rating: N/A |
Russell, maybe you could benefit from the use of a desulfator. That's what I would try. Tom Caffrey PD4106-2576 Suncatcher Ketchikan, Alaska | ||
| Russell Beam (Rcbeam)
Registered Member Username: Rcbeam Post Number: 46 Registered: 11-2009 Posted From: 74.131.50.239 Rating: N/A |
Possibly so Tom, but then again, these batteries are 7 years old... not sure I really want to fool with trying to resurrect them for how long??? I've many more dragons to slay with the bus before it turns too cold to do anything. It seems like one step forward and two or three back. |
